use rust-encoding to correctly(?) set utf16 strings for cmdline args as expected
implement cef_string_utf8_to_utf16()
implement cef_string_utf16_to_utf8()
remove rust-encoding usage from cef
use enc.len() for cef_string_utf8_to_utf16 conversion size
handle str::from_utf8() failure, improve return calls
auto merge of #3659 : zmike/servo/embedding-encoding, r=SimonSapin

r+ @larsbergstrom ?