array.c: eql? identity check if immediate value

[dwfait]

I previously opened this: #1200

This PR introduces similar performance improvements for 'immediate' (Fixnum, floats, etc..) values, whilst not changing behaviour for Float::NAN or objects with redefined eql? methods.

caveat: This does change behaviour if you monkey patch the eql? method for an immedate value class, but if you're doing that, I imagine Array#eql? is going to be low down on your list of problems.

Benchmarks

Marked improvement for arrays of immediate values: https://gist.github.com/dwfait/da225dcc4687dcaff2a5

No apparent degradation of performance for arrays of objects / non-immediate values: https://gist.github.com/dwfait/9add4b6442897d68a2cd

Float::NAN

As mentioned, in this PR treatment of Float::NAN is as before:

irb(main):001:0> [Float::NAN].eql? [Float::NAN]
=> false

However, I did not create a test for this, because Matz has stated comparison of NaN to NaN is undefined: https://bugs.ruby-lang.org/issues/1720

I have however included a test which I believe addresses the core of that issue - of objects where eql? will return false, even to itself.

[k0kubun]

It seems to have a conflict now. Could you rebase this from master?

[k0kubun]

Let me close this as it has not been updated for a while. Please reopen this after resolving conflicts. Thanks.