Move Hash specs to rely less on shared examples

[Earlopain]

See #1364

This is mostly a mechanical translation. A few things of interest:

Two shared specs weren't actually shared. Hash#index used to be an alias for Hash#key but was removed at some point. Hash#values_at had shared specs which never were actually shared.

Tests for Hash#[]= used send. I explicitly added back a test usign send to assert the return value.

I cleaned up requires, each test file can be run in isolation.

For ../mspec/bin/mspec core/hash:

Before:
69 files, 633 examples, 1176 expectations, 0 failures, 0 errors, 0 tagged

After:
69 files, 558 examples, 1026 expectations, 0 failures, 0 errors, 0 tagged

So 75 fewer specs, 150 fewer assertions