Add support for compiling a lookup

[superfell]

This separates out the tokenizing / step generation into a reusable result that you can then use to do many lookup operations. As most of the lookup time is spent generating tokens/steps this can be a big win, e.g.

goos: darwin
goarch: amd64
pkg: github.com/oliveagle/jsonpath
BenchmarkJsonPathLookupCompiled-8   	20000000	        89.5 ns/op
BenchmarkJsonPathLookup-8           	 1000000	      1733 ns/op

[oliveagle]

👍