optimisation of uri_escape

A little optimisation of uri_escape - we can convert uint8 to hex representation without sprintf overhead.
For performance test I used this script: 
time perl -I lib/ -I ./blib/arch -e 'use URI::Escape::XS qw/uri_escape/; my $data = join "", map {chr($_)} 0..255; for (1..1e6) {uri_escape($data)}'

old time:
real	0m34.706s
user	0m34.676s
sys	0m0.024s

new time:
real	0m2.167s
user	0m2.159s
sys	0m0.006s