Add evaluation benchmarks

[markphelps]

➜ make bench
>> running benchmarks
go test -v -bench=. ./... -run=XXX

goos: darwin
goarch: amd64
pkg: github.com/markphelps/flipt/storage
BenchmarkEvaluate_SingleVariantDistribution/match_string_value-4                    4494            242572 ns/op           24074 B/op        486 allocs/op
BenchmarkEvaluate_SingleVariantDistribution/no_match_string_value-4                 6280            187252 ns/op           19620 B/op        390 allocs/op
BenchmarkEvaluate_SingleVariantDistribution/no_match_just_bool_value-4              6550            202475 ns/op           18034 B/op        381 allocs/op
BenchmarkEvaluate_SingleVariantDistribution/no_match_just_string_value-4            6272            181950 ns/op           17939 B/op        381 allocs/op
BenchmarkEvaluate_RolloutDistribution/match_string_value_-_variant_1-4              5026            232469 ns/op           22742 B/op        478 allocs/op
BenchmarkEvaluate_RolloutDistribution/match_string_value_-_variant_2-4              5065            232568 ns/op           22808 B/op        478 allocs/op
BenchmarkEvaluate_RolloutDistribution/no_match_string_value-4                       6913            171050 ns/op           17217 B/op        365 allocs/op
BenchmarkEvaluate_NoConstraints/match_no_value_-_variant_1-4                        5432            218564 ns/op           21207 B/op        456 allocs/op
BenchmarkEvaluate_NoConstraints/match_no_value_-_variant_2-4                        4710            221971 ns/op           21206 B/op        456 allocs/op
BenchmarkEvaluate_NoConstraints/match_string_value_-_variant_2-4                    5379            224047 ns/op           22427 B/op        468 allocs/op