Remove the tokeniser hack that prevents compile-time vivification of %stash::

Remove the tokeniser hack that prevents compile-time vivification of %stash::

This was put in to ensure that defined %stash:: continued to return false after
the implementation of hashes was changed, such that stashes were always defined.
defined %stash:: is deprecated.

This reverts the tokeniser changes of adc51b978ed1b2e9d4512c9bfa80386ac917d05a,
76138434928a968a390c791aec92e5f00017d01d,
d6069db2e52f58ef65bf59f2fd453604270d2205 and part of
9bde8eb087a2c05d4c8b0394a59d28a09fe5f529, and updates the tests added with those
commits to reflect the restored (but as yet unreleased) behaviour.

I don't think that this should be merged to blead until after 5.12.0 ships,
with the enabled deprecation warnings on defined %hash, as it changes subtle
behaviour that all current released stable perls accept without warning.