Optimize Base.valid16?/2

[sabiwara]

EDIT: the benchmark below was checking for an invalid string - discard and see sabiwara/elixir_benches@af34130 (still quite a nice speedup)

• Using and instead of relying on a global rescue has a significant impact
• Using a guard instead of a local rescue to check if the char is a non-nil member of a tuple also has almost as big of an impact

I haven't benched/done other functions yet, wanted to first share this one and wait for the feedback.

Reusing @whatyouhide 's big_string benchmark:

##### With input big string #####
Name                      ips        average  deviation         median         99th %
bool_plus_guard       11.97 M       83.56 ns ±13110.49%       20.90 ns      104.20 ns
bool                   4.72 M      212.01 ns ±19289.61%          84 ns         250 ns
original               1.31 M      765.72 ns  ±3523.35%         500 ns        1417 ns

Comparison: 
bool_plus_guard       11.97 M
bool                   4.72 M - 2.54x slower +128.46 ns
original               1.31 M - 9.16x slower +682.17 ns

Memory usage statistics:

Name               Memory usage
bool_plus_guard            40 B
bool                      288 B - 7.20x memory usage +248 B
original                 1216 B - 30.40x memory usage +1176 B

**All measurements for memory usage were the same**

Reduction count statistics:

Name            Reduction count
bool_plus_guard               8
bool                          8 - 1.00x reduction count +0
original                     73 - 9.13x reduction count +65

**All measurements for reduction count were the same**

[sabiwara]

Thx! Will merge this one and send a follow-up for other bases