Debug build fixes

As mentioned in #60, there were some failures in debug builds.  This PR fixes them up and optimizes NanRawString() as a side effect.

With joyent/node@0e3d669:

```
$ node-gyp --debug --nodedir $HOME/src/master rebuild
# <elided>

$ $HOME/src/master/out/Debug/node node_modules/tap/bin/tap.js js/*.js
ok js/asyncworker-test.js ............................... 3/3
ok js/fromv8string-test.js ............................ 24/24
ok js/multifile-test.js ................................. 3/3
ok js/optionvalues-test.js .............................. 3/3
ok js/persistent.js ..................................... 8/8
ok js/returnemptystring-test.js ......................... 4/4
ok js/returnnull-test.js ................................ 4/4
ok js/returnundefined-test.js ........................... 4/4
ok js/returnvalue-test.js ............................... 4/4
ok js/settergetter-test.js .............................. 5/5
ok js/symbols-test.js ................................... 3/3
total ................................................. 65/65
```