Fixing memoize edge cases with predicate and bang methods

[jwoertink]

Purpose

Fixes #1396

Description

This allows you to memoize predicate and bang methods.

# this is now possible
memoize def current_user? : User?
  UserQuery.new.id(1).first?
end

Checklist

• - An issue already exists detailing the issue/or feature request that this PR fixes
• - All specs are formatted with crystal tool format spec src
• - Inline documentation has been added and/or updated
• - Lucky builds on docker with ./script/setup
• - All builds and specs pass on docker with ./script/test