JSON parser: Refactor expectStringUtf8() to generic expectString().

The new implementation is as fast as old expectStringAscii() when the string does not contain any \u1234 escape sequences, and much faster than old expectStringUtf8() - as we don't do useless conversion UTF8 -> Unicode -> UTF8.

Note: If https://github.com/jeremybarnes/jml/pull/4 becomes merged, then all the helper functions (expectJsonStringChar, codepointFromHex, fromHex) will be useless - as ML::expectJsonStringChar() can be used instead.