Optimize IN comparion by using a map instead of slice when possible

[Richtermeister]

This PR aims to optimize IN expressions with static clauses. The example benchmark a in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10) included is rather small but illustrates the point, especially since allocations grow in proportion to number of entries involved. In production we have expressions against 40k+ IN entries, which massively benefits from this optimization.

goos: darwin
goarch: amd64
pkg: github.com/govaluate
cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
BenchmarkIn

Before:
BenchmarkIn-16    	 1050931	      1106 ns/op	    1056 B/op	      17 allocs/op

After:
BenchmarkIn-16    	 4521502	       269.5 ns/op	     360 B/op	       4 allocs/op