Don't precalculate `#hash` method

Intentional trade-off. This will make using Value classes
as _hash keys_ a bit slower for repeated hash insertions. But
faster for constructor calls.

Our experience has been that this is definitely the right
trade-off for us: we spend a lot of time in `.new` and `#with`
calls, and very little time inserting Value classes as hash
keys or Set elements.

Version 1.8.0

Version 1.7.1

Version 1.7.0

Version 1.6.0

Regenerate gemspec for version 1.5.0

Regenerate gemspec for version 1.4.0

Regenerate gemspec for version 1.3.0

Regenerate gemspec for version 1.2.1

Regenerate gemspec for version 1.2.0