Make the inputs const since we never write to the data it points to.

Move the declaration of i to each for loop. This makes it clear there's
no interaction with i outside each loop. It also makes it so compilers
wont complain with -Wshadow.

2x to 5x speed-up at the cost of ~600B of RAM. Compile with -DDEFLATE_WITH_LUT

this library is now fully RFC 1950 and RFC 1951 compliant