Remove x2p

This removes find2perl, s2p and a2p from core. They have all been
released to CPAN as separate distributions.
UV casting to avoid intermediate sign extension.

[perl #121746]

Fix for Coverity perl5 CIDs 29069, 29070, 29071:
Unintended sign extension: ... ... if ... U8 (8 bits unsigned) ... 32
bits, signed ...  64 bits, unsigned ... is greater than 0x7FFFFFFF,
the upper bits of the result will all be 1.