Pass a JSContext to ToJSValConvertible::to_jsval.
Allow passing options to FromJSValConvertible::from_jsval.
Replace str_to_jsval and domstring_to_jsval by a ToJSValConvertible implementation.
Replace jsval_to_str and jsval_to_domstring with a FromJSValConvertible implementation.
auto merge of #1876 : Ms2ger/servo/JSValConvertible-strings, r=jdm

Hat tip to @eddyb and @huonw for most of the clever bits of this PR.