Fix length argument to v8::String::NewFromUtf8().

Rust strings are not nul-terminated, v8.rs needs to explicitly pass the
string length to V8.

Bug introduced in commit 4059d0b ("Call V8 API directly.") and at least
partially responsible for the failing native_api_call test.
Implement Show for V8 data types.
Implement PartialEq for V8 data types.
Rewrite fmt() and to_option() in terms of raw_ptr().