Implement shrink_to_fit method on StringInterner

[artemshein]

It could be a good idea to be able to free unused memory after all strings are interned.

[coveralls]

Coverage decreased (-96.8%) to 0.0% when pulling e428f0e on artemshein:master into 5cf47d0 on Robbepop:master.

[Robbepop]

Thank you for the PR!
I haven't thought about this use case but it might be useful to some users.

Do you know why coveralls reports 0% coverage decrease?

[artemshein]

I have no idea)

[Robbepop]

I found out the reason but it is strange that it did not break earlier ...
Tomorrow I will take some time and merge your PR! ;)

[coveralls]

Coverage remained the same at 96.016% when pulling 2c183f2 on artemshein:master into 069293b on Robbepop:master.

[coveralls]

Coverage remained the same at 96.016% when pulling 2c183f2 on artemshein:master into 069293b on Robbepop:master.

[coveralls]

Coverage remained the same at 96.016% when pulling 2c183f2 on artemshein:master into 069293b on Robbepop:master.

[codecov-io]

Codecov Report

Merging #3 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master       #3   +/-   ##
=======================================
  Coverage   96.01%   96.01%           
=======================================
  Files           3        3           
  Lines         251      251           
=======================================
  Hits          241      241           
  Misses         10       10

Impacted Files	Coverage Δ
src/lib.rs	92.63% <ø> (ø)	⬆️

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