string_bytes: don't use named imports in header

It imports the definition into every source file that includes
string_bytes.h, as evidenced by the build suddenly breaking left
and right because of missing Handle/Local/String/Value imports.
src: remove unused import in src/node_os.cc
src: const-ify variables in src/node_crypto*

No functional changes, just some code tightening. Clean up some style
inconsistencies while we are here.
crypto: simplify DH modp group name matcher

* Use ARRAY_SIZE() rather than scanning until we hit a NULL entry.
* Fix `-fsigned-char -Wnarrowing` compiler warnings. Harmless but
  numerous and annoying.
* Static-ify the modp_group and mod_groups arrays.
* Const-ify the modp_groups array.
deps: upgrade libuv to joyent/libuv@4bdb7d8

Non-release upgrade so pending patches can land.
string_bytes: export GetExternalParts

The method is useful elsewhere when needing to check if external and
grab data.
fs: write strings directly to disk

Prior, strings would first be converted to a Buffer before being written
to disk. Now the intermediary step has been removed.

Other changes of note:

* Class member "must_free" was added to req_wrap so to track if the
  memory needs to be manually cleaned up after use.
* External String Resource support, so the memory will be used directly
  instead of copying out the data.
* Docs have been updated to reflect that if position is not a number
  then it will assume null. Previously it specified the argument must be
  null, but that was not how the code worked. An attempt was made to
  only support == null, but there were too many tests that assumed !=
  number would be enough.
* Docs update show some of the write/writeSync arguments are optional.
util: don't throw on circular %j input to format()

Don't throw an exception when the argument to %j is an object that
contains circular references, it's not helpful. Catch the exception
and return the string '[Circular]'.
node: add inTick and lastThrew to infoBox

To prevent all unnecessary calls to JS from MakeCallback, the remaining
two immediate return variables inTick and lastThrew have been added to
infoBox. Now MakeCallback should never need to call into JS unless it
absolutely has to.

Also removed Tock. Performance tests showed it was at least as fast or
faster than using a normal object, and this is more readable.