Small clean-ups to Regexp module.

- use .length instead of .size on string to be consistent and simplify
  logic surrounding some of those calls

- fix regexp:error() to return a string (was returning a c_string)

- fix _cast() -- did not work in the non-local case (I suspect there
  is no test case for this)

- added note on stringPart regarding possible use of c_string
Merge branch 'master' of github.com:chapel-lang/chapel into test

Conflicts:
	modules/standard/Regexp.chpl
Merge pull request #110 from sungeunchoi/test

Small cleanups to Regexp module

[reviewed by Ben]

- Use string/c_string.length to be consistent throughout the file (it also reduces the work of function resolution and inlining slightly since .size calls .length and both are inlined)
- Simplify the stringPart specific logic surrounding the .length calls since now we use the same field name
- Comment a couple of places that seem like they needed stringPart logic but
- Add a comment for stringPart regarding the possible use of c_string rather than string
- Fix regexp:error() to return a string as the return type is specified (previously was returning c_string)
- Fix _cast() for multi-locale case (I suspect there is no test case for this)