bug: $x.$number() incorrectly returns no value

[tommy]

The . syntax introduces a "context" variable named $. Some built-in functions ($string, $number, $abs, etc) are meant to use the value of $ as the first parameter if no explicit arguments are provided. This is probably a discrepancy with our other functions as well.

/* in the Javascript reference implementation: */
( $x := "123"; $number($x) ) /* evals to 123 */ ,
( $x := "123"; $x.$number($) ) /* evals to 123 */ ,
( $x := "123"; $x.$number() ) /* evals to 123 */ ,

/* in jsonata-rs: */
( $x := "123"; $number($x) ) /* evals to 123 */ ,
( $x := "123"; $x.$number($) ) /* evals to 123 */ ,
( $x := "123"; $x.$number() ) /* returns no match */ ,

Note that this behavior is (as far as I can tell) specific to each function. That is, it's not the case that every function invoked in this way gets the context parameter injected. Eg:

/* Javascript */
( $f := function($arg) { $arg } ; $x := "123" ; $x.$f() ) /* returns no match */ ,
( $f := function($arg) { $arg } ; $x := "123" ; $f($x) ) /* returns "123" */ ,