Implement name munging and solve foreign subroutine edge cases

This patch that simultaneously adds a new feature and fixes a few bugs
was hard to split up, so I'm commiting this all in one go.

This solves and tests for cases where we tried to call
constant.pm-generated subroutines and would previously fail to correctly
unpack their values from the stash (since constant.pm cheats and doesn't
actually create subs).

This also implements an unrelated feature where we can now munge the
internal names we have for constants we're generating. See the
documentation for what this is for.
Test Coverage: Test for passing no "options"

I'm testing coverage with:

    (git clean -dxf; dzil build; cd Constant-Export-Lazy-*; perl Makefile.PL && cover -test)
Test Coverage: Test for invalid wrapping of existing import
Test Coverage: Test for clobbering existing import sub
Test Coverage: Test for importing invalid constants
Test Coverage: Test for declaring invalid constants
Test Coverage: Test for broken override
Test Coverage: Test for broken after
Test Coverage: Test for re-fetching sub without wrap_existing_import

This actually didn't end up helping coverage, but I'm keeping the test
anyway to test for this case.
Changes: Release notes for 0.04
Copyedit some of the documentation to a hopefully better state
Micro-optimize and save space on $wrap_existing_import when it's false

We only ever check if this is true, so just set it to undef when it's
false instead of 0.