Add additional cases for String#to_i benchmarks
Add spec for turning string into Bignum for String#to_i
Improve performance of String#to_i for Fixnum sized numbers

This improves performance by using a native_int directly instead of
going through Fixnum::mul for small numbers.

Before:

=== bin/rbx ===
                     23003211.6 (±2.9%) i/s -  114623120 in   4.987875s
                      8712371.5 (±3.0%) i/s -   43457940 in   4.993010s
                     22075592.9 (±3.5%) i/s -  110172960 in   4.998188s
                      9270667.3 (±1.7%) i/s -   46350018 in   5.001216s
                     26227093.3 (±2.4%) i/s -  130860198 in   4.992595s
                     21890007.0 (±2.0%) i/s -  109247625 in   4.992789s
                      2778600.2 (±5.4%) i/s -   13821408 in   4.996930s

After:

=== bin/rbx ===
                     26079388.8 (±2.5%) i/s -  129962508 in   4.986793s
                     14049298.6 (±2.4%) i/s -   70156344 in   4.996964s
                     24275838.8 (±4.2%) i/s -  120887196 in   4.991455s
                     13268209.3 (±3.4%) i/s -   66240912 in   4.999549s
                     28462400.4 (±2.1%) i/s -  141973920 in   4.990660s
                     24266645.9 (±3.2%) i/s -  120924700 in   4.988763s
                     12897795.2 (±2.1%) i/s -   64409609 in   4.996203s
Also expose *_gvl functions in 1.9 mode

Apparently this also works with MRI 1.9.

Fixes #2566