Implement partial function application

[thekid]

This pull request implements limited support for partial function applications (PFA) for PHP 7.4 - 8.4:

In a nutshell

PFA builds on top of callable closure syntax (e.g. $f= strlen(...)) by adding a new placeholder representing a single argument, the question mark (?):

$replace= str_replace('test', 'success', ?); // $replace is a closure
$result= $replace('A test.');                // $result equals "A success."

Implementation

• Rewrite str_replace('test', 'ok', ?) to fn($arg) => str_replace('test', 'ok', $arg)
• Make evaluation order consistent with native implementation by evaluating non-constant arguments at declaration
• Make this work for new expressions (unlike PHP, which does not support this!)
• Implement optimizations for pipeline operator (emit "test" |> strlen(?) as strlen("test"))

Limitations

• Support by-ref arguments - this would require runtime reflection checks, resulting in a significant performance hit
• Invoking with named parameters - same as above, would require runtime reflection
• Parameter and return type anntotations - same as above, would require runtime reflection
• Variadic placeholder in the middle of arguments (e.g. $c = stuff(1, ?, ..., p: ?, f: 3.14);)

See also

• https://externals.io/message/127781 (Announcement)
• https://wiki.php.net/rfc/partial_function_application_v2 (RFC)
• Partials v2 arnaud-lb/php-src#12 (POC)
• Add syntactic support for partial function application ast#57 (Dependency)

[thekid]

This PR is now in a pretty complete state (with the limitations listed above), let's wait and see how the vote on the PHP RFC goes, and then feature-test for parity once the RFC's implementation has stabilized.