Test string pointers validity and explain safety

[lo48576]

wrt #10 (comment).

Added StringInterner::assert_internal_str_refs_validity() and helper function StringInterner::max_capacity(), that are enabled only in test bulid.
This detects dangling pointer and overlooked interned strings, and panics if something wrong.
Using this assertion in tests, it will be possible to detect dangling pointers once it appears.

It will be hard to test every possible case, so I added a test for typical case in which the pointer can be modified or invalidated: when interning new string (and specifically when reallocation happens), and when being cloned.
It is tests::internal_str_refs_validity::intern_reallocate_clone().

Additionally, I added a comment to explain why the uses of InternalStrRefare safe, for future contributors and reviewers.

[coveralls]

Coverage decreased (-1.1%) to 93.778% when pulling da8b0f3 on lo48576:feature/assert-string-pointer-validity into 9ca8921 on Robbepop:master.

[codecov-io]

Codecov Report

Merging #12 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   94.43%   94.43%           
=======================================
  Files           2        2           
  Lines         413      413           
=======================================
  Hits          390      390           
  Misses         23       23

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[Robbepop]

Thanks for the huge comment describing the safety guarantees of the unsafe code parts.
I am not sure whether it should be a normal // comment of a doc comment (and where to put it then).
Also it was sometimes hard to read due to some writing mistakes here and there. Please fix them and we can merge.
Rest looks fine.

[lo48576]

I used normal // comment because it is documentation for developers (and code readers), but not for usual library users.
(Internal uses of unsafes are not visible to users, and they would have nothing to do with it when they use the crate.)

[lo48576]

Fixed some sentences, but I won't notice some kind of mistakes and unnatural English, because it's not my native tongue...
I'm happy if you could point out such parts to be fixed 🙏

[lo48576]

Closing this as outdated.