Document bit_size in match-specs and allow in fun2ms

[gomoripeti]

It is already allowed in match-specs.

[sverker]

May I suggest a rebase on maint for OTP-21.2.

git rebase --onto maint master

[gomoripeti]

sorry, I chose the wrong target branch, now it should be maint

[sverker]

I see that byte_size is missing totally, but that would be another PR.

[gomoripeti]

yes, byte_size is not even there in the PAM program interpreter. I wouldn't have dared to be so lazy to not add tests if I'd modified that part. And bit_size is good enough for most of (my) use-cases with bytestrings/Elixir strings.