Inline round functions
Inline round functions

More that removing function call overhead, this lets us replace r[0]
with r0 which removes all the bounds checks.  This gives us a
considerable speedup.

This takes us from ~35% slower than ebfe/rc2 to ~25% faster.

benchmark                old ns/op     new ns/op     delta
BenchmarkEncrypt         610           365           -40.16%
BenchmarkDecrypt         594           329           -44.61%

dgryski/go-rc2