feat: allow any string or integer in array key

[romm]

The following array key types are now accepted and will work properly with the mapper:

$mapper->map("array<'foo'|'bar', string>", ['foo' => 'foo']);
$mapper->map('array<42|1337, string>', [42 => 'foo']);
$mapper->map('array<positive-int, string>', [42 => 'foo']);
$mapper->map('array<negative-int, string>', [-42 => 'foo']);
$mapper->map('array<int<-42, 1337>, string>', [42 => 'foo']);
$mapper->map('array<non-empty-string, string>', ['foo' => 'foo']);
$mapper->map('array<class-string, string>', ['SomeClass' => 'foo']);

Fixes #312