fixed #2 - use backward-compatible isALNUM

isWORDCHAR was introduced somewhere in 5.13
Mojo::Base::XS requirement is only 5.10
match Mojo::Base accessor name restrictions
return new SV in (possible) lvalue context

See bug #82689 Class::XSAaccessor RT
https://rt.cpan.org/Public/Bug/Display.html?id=82689
ported TARG optimization from Class::XSAccessor

Ok, I've just shamelessly stolen it.
As the rest of the code, actually.

The original intention is to replace sv_2mortal
with sv_setsv => TARG

This solution still requires benchmarking.
cleanup
exception instead of warning if Mojo::Base::XS failed to store the value
updated changes, bumped version