Test surrogate pair encoding
Use unicode notation

:tophat: @aroben
Encode strings as CESU-8
1.9.3 does not support NULL for `from` in str_conv_enc
Improved CESU-8 encoding tests/support
Fix a bug where the error message was garbage collected
Merge branch 'cesu8'
Add `struct state`:

Instead of passing duk_context around we now wrap it in a `struct state`
where we can add more variables/state that we need.