⚡ Use stack-allocated span for search movegen

[eduherminio]

Elo   | -0.46 +- 4.01 (95%)
SPRT  | 8.0+0.08s Threads=1 Hash=32MB
LLR   | -2.27 (-2.25, 2.89) [0.00, 5.00]
Games | N: 17486 W: 5311 L: 5334 D: 6841
Penta | [662, 2010, 3404, 2023, 644]
https://openbench.lynx-chess.com/test/31/

Elo   | 4.11 +- 5.59 (95%)
SPRT  | 40.0+0.40s Threads=1 Hash=128MB
LLR   | 2.91 (-2.25, 2.89) [-5.00, 0.00]
Games | N: 8284 W: 2364 L: 2266 D: 3654
Penta | [236, 952, 1683, 1020, 251]
https://openbench.lynx-chess.com/test/33/

`BenchUCI` BDN benchmark: noticeable improvement

Before:

BenchmarkDotNet v0.13.11, Ubuntu 22.04.3 LTS (Jammy Jellyfish)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.5[310](https://github.com/lynx-chess/Lynx/actions/runs/7334435367/job/19971116018#step:7:311)3), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

| Method             | Mean     | Error    | StdDev   | Allocated |
|------------------- |---------:|---------:|---------:|----------:|
| Bench_DefaultDepth | 540.0 ms | 11.21 ms | 33.07 ms | 329.53 MB |


BenchmarkDotNet v0.13.11, Windows 10 (10.0.20348.2159) (Hyper-V)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

| Method             | Mean     | Error    | StdDev   | Median   | Gen0      | Gen1      | Gen2      | Allocated |
|------------------- |---------:|---------:|---------:|---------:|----------:|----------:|----------:|----------:|
| Bench_DefaultDepth | 442.3 ms | 10.02 ms | 28.76 ms | 453.8 ms | 5000.0000 | 1000.0000 | 1000.0000 | 329.54 MB |

BenchmarkDotNet v0.13.11, macOS Monterey 12.7.2 (21G1974) [Darwin 21.6.0]
Intel Xeon CPU E5-1650 v2 3.50GHz (Max: 3.34GHz), 1 CPU, 3 logical and 3 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX

| Method             | Mean     | Error    | StdDev   | Median   | Gen0       | Gen1      | Gen2      | Allocated |
|------------------- |---------:|---------:|---------:|---------:|-----------:|----------:|----------:|----------:|
| Bench_DefaultDepth | 704.5 ms | 25.91 ms | 73.08 ms | 669.5 ms | 13000.0000 | 2000.0000 | 1000.0000 | 329.57 MB |

After:
Significatively better in Windows, better in Linux, cannot really compare in macOS due to different machine specs

BenchmarkDotNet v0.13.11, Ubuntu 22.04.3 LTS (Jammy Jellyfish)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.5[310](https://github.com/lynx-chess/Lynx/actions/runs/7334455240/job/19971159680#step:7:311)3), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

| Method             | Mean     | Error    | StdDev   | Allocated |
|------------------- |---------:|---------:|---------:|----------:|
| Bench_DefaultDepth | 519.8 ms | 11.03 ms | 32.19 ms | 313.33 MB |

BenchmarkDotNet v0.13.11, Windows 10 (10.0.20348.2159) (Hyper-V)
AMD EPYC 7763, 1 CPU, 4 logical and 2 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2

| Method             | Mean     | Error   | StdDev  | Gen0      | Gen1      | Gen2      | Allocated |
|------------------- |---------:|--------:|--------:|----------:|----------:|----------:|----------:|
| Bench_DefaultDepth | 375.1 ms | 4.23 ms | 3.30 ms | 4000.0000 | 1000.0000 | 1000.0000 | 313.34 MB |


BenchmarkDotNet v0.13.11, macOS Monterey 12.7.2 (21G1974) [Darwin 21.6.0]
Intel Core i7-8700B CPU 3.20GHz (Max: 3.19GHz) (Coffee Lake), 1 CPU, 4 logical and 4 physical cores
.NET SDK 8.0.100
  [Host]     : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.0 (8.0.23.53103), X64 RyuJIT AVX2


| Method             | Mean     | Error    | StdDev   | Gen0       | Gen1      | Gen2      | Allocated |
|------------------- |---------:|---------:|---------:|-----------:|----------:|----------:|----------:|
| Bench_DefaultDepth | 777.0 ms | 33.12 ms | 96.09 ms | 10000.0000 | 2000.0000 | 1000.0000 | 313.36 MB |

Perft: ~10% speedup

Before:

Run dotnet run -c Release --no-build "position fen r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq - 0 1" "perft 5" "quit"
Lynx 1.1.0 by Eduardo Caceres
Depth:	5
Nodes:	193690690
Time:	10.05 s
nps:	19.266 Mnps

After:
+11.41% speed

Run dotnet run -c Release --no-build "position fen r3k2r/p1ppqpb1/bn2pnp1/3PN3/1p2P3/2N2Q1p/PPPBBPPP/R3K2R w KQkq - 0 1" "perft 5" "quit"
Lynx 1.1.0 by Eduardo Caceres
Depth:	5
Nodes:	193690690
Time:	9.02 s
nps:	21.464 Mnps