hex decode table; increases footprint but a bit faster
Merge branch 'hex-decode-table'

Use a 256-byte table for hex decoding.  This affects several places
where hex values were decoded with an if-else ladder.  There's some
net footprint increase.  Performance increases are mostly limited to
duk_decode_hex() API call and Duktape.dec('jx', ...) calls.  Elsewhere
the performance matters less.