[JRuby] Optimize scan(): Use strBL.getBegin() + curr instead of currPtr() (#109)

- before: #106

## Why?

Because they are identical.


https://github.com/ruby/strscan/blob/d31274f41b7c1e28f23d58cf7bfea03baa818cb7/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java#L267-L268


https://github.com/ruby/strscan/blob/d31274f41b7c1e28f23d58cf7bfea03baa818cb7/ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java#L359-L361

## Benchmark

It shows String as a pattern is 2.33x faster than Regexp as a pattern.

```
$ benchmark-driver benchmark/check_until.yaml
Warming up --------------------------------------
              regexp     7.421M i/s -      7.378M times in 0.994235s (134.75ns/i)
          regexp_var     7.302M i/s -      7.307M times in 1.000706s (136.95ns/i)
              string    12.715M i/s -     12.707M times in 0.999388s (78.65ns/i)
          string_var    13.575M i/s -     13.533M times in 0.996914s (73.66ns/i)
Calculating -------------------------------------
              regexp     8.287M i/s -     22.263M times in 2.686415s (120.67ns/i)
          regexp_var    10.180M i/s -     21.905M times in 2.151779s (98.23ns/i)
              string    20.148M i/s -     38.144M times in 1.893226s (49.63ns/i)
          string_var    23.695M i/s -     40.726M times in 1.718753s (42.20ns/i)

Comparison:
          string_var:  23694846.7 i/s
              string:  20147598.6 i/s - 1.18x  slower
          regexp_var:  10180018.3 i/s - 2.33x  slower
              regexp:   8287384.8 i/s - 2.86x  slower
```

Author: naitoh

ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java

@@ -263,20 +263,18 @@ private IRubyObject extractBegLen(Ruby runtime, int beg, int len) {
     private IRubyObject scan(ThreadContext context, IRubyObject regex, boolean succptr, boolean getstr, boolean headonly) {
         final Ruby runtime = context.runtime;
         check(context);
-
-        ByteList strBL = str.getByteList();
-        int strBeg = strBL.getBegin();
-
         clearMatched();
 
         if (restLen() < 0) {
             return context.nil;
         }
 
+        ByteList strBL = str.getByteList();
+        int currPtr = strBL.getBegin() + curr;
+
         if (regex instanceof RubyRegexp) {
             pattern = ((RubyRegexp) regex).preparePattern(str);
 
-            int currPtr = currPtr();
             int range = currPtr + restLen();
 
             Matcher matcher = pattern.matcher(strBL.getUnsafeBytes(), matchTarget(), range);
@@ -311,12 +309,12 @@ private IRubyObject scan(ThreadContext context, IRubyObject regex, boolean succp
             int patternSize = patternBL.realSize();
 
             if (headonly) {
-                if (ByteList.memcmp(strBL.unsafeBytes(), strBeg + curr, patternBL.unsafeBytes(), patternBL.begin(), patternSize) != 0) {
+                if (ByteList.memcmp(strBL.unsafeBytes(), currPtr, patternBL.unsafeBytes(), patternBL.begin(), patternSize) != 0) {
                     return context.nil;
                 }
                 setRegisters(patternSize);
             } else {
-                int pos = StringSupport.index(strBL, patternBL, strBeg + curr, patternEnc);
+                int pos = StringSupport.index(strBL, patternBL, currPtr, patternEnc);
                 if (pos == -1) {
                     return context.nil;
                 }