Only determine needed emulators based on PHP version once, and
add an adaptor that allows treating forward and reverse emulation
the same.
Previously the isEmulationNeeded() check was too conservative,
as it also considered emulators that are not relevant for the
version. Though possibly that check should just be dropped
altogether.
a) Don't check for !== false, preg_match() return 0 if there is
no match. This effectively means that the check was always true.
b) Check for hex characters only if there's an 0x prefix. 1_1 is
very likely a numeric separator, but a_b might well be part of
an identifier.
Fixes#639.
