(https://github.com/ruby/strscan/pull/106)

It supports non-head match cases such as StringScanner#scan_until.

If we use a String as a pattern, we can improve match performance.
Here is a result of the including benchmark.

## CRuby

It shows String as a pattern is 1.18x faster than Regexp as a pattern.

```
$ benchmark-driver benchmark/check_until.yaml
Warming up --------------------------------------
              regexp     9.403M i/s -      9.548M times in 1.015459s (106.35ns/i)
          regexp_var     9.162M i/s -      9.248M times in 1.009479s (109.15ns/i)
              string     8.966M i/s -      9.274M times in 1.034343s (111.54ns/i)
          string_var    11.051M i/s -     11.190M times in 1.012538s (90.49ns/i)
Calculating -------------------------------------
              regexp    10.319M i/s -     28.209M times in 2.733707s (96.91ns/i)
          regexp_var    10.032M i/s -     27.485M times in 2.739807s (99.68ns/i)
              string     9.681M i/s -     26.897M times in 2.778397s (103.30ns/i)
          string_var    12.162M i/s -     33.154M times in 2.726046s (82.22ns/i)

Comparison:
          string_var:  12161920.6 i/s
              regexp:  10318949.7 i/s - 1.18x  slower
          regexp_var:  10031617.6 i/s - 1.21x  slower
              string:   9680843.7 i/s - 1.26x  slower
```

## JRuby

It shows String as a pattern is 2.11x faster than Regexp as a pattern.

```
$ benchmark-driver benchmark/check_until.yaml
Warming up --------------------------------------
              regexp     7.591M i/s -      7.544M times in 0.993780s (131.74ns/i)
          regexp_var     6.143M i/s -      6.125M times in 0.997038s (162.77ns/i)
              string    14.135M i/s -     14.079M times in 0.996067s (70.75ns/i)
          string_var    14.079M i/s -     14.057M times in 0.998420s (71.03ns/i)
Calculating -------------------------------------
              regexp     9.409M i/s -     22.773M times in 2.420268s (106.28ns/i)
          regexp_var    10.116M i/s -     18.430M times in 1.821820s (98.85ns/i)
              string    21.389M i/s -     42.404M times in 1.982519s (46.75ns/i)
          string_var    20.897M i/s -     42.237M times in 2.021187s (47.85ns/i)

Comparison:
              string:  21389191.1 i/s
          string_var:  20897327.5 i/s - 1.02x  slower
          regexp_var:  10116464.7 i/s - 2.11x  slower
              regexp:   9409222.3 i/s - 2.27x  slower
```

See:
https://github.com/jruby/jruby/blob/be7815ec02356a58891c8727bb448f0c6a826d96/core/src/main/java/org/jruby/util/StringSupport.java#L1706-L1736

---------

https://github.com/ruby/strscan/commit/f9d96c446a



Co-authored-by: Sutou Kouhei <kou@clear-code.com>