Manipulating stashes (Perl's symbol tables) is occasionally necessary, but
incredibly messy, and easy to get wrong. This module hides all of that
behind a simple API.

NOTE: Most methods in this class require a variable specification that
includes a sigil. If this sigil is absent, it is assumed to represent the
IO slot.

Due to limitations in the typeglob API available to perl code, and to
typeglob manipulation in perl being quite slow, this module provides two
implementations - one in pure perl, and one using XS. The XS implementation
is to be preferred for most usages; the pure perl one is provided for cases
where XS modules are not a possibility. The current implementation in use
can be set by setting '$ENV{PACKAGE_STASH_IMPLEMENTATION}' or
'$Package::Stash::IMPLEMENTATION' before loading Package::Stash (with the
environment variable taking precedence), otherwise, it will use the XS
implementation if possible, falling back to the pure perl one.