Intern bools to avoid allocation on bool to interface{}

[prashantv]

This doesn't affect any functionality, just reduces allocations and improves performance.

Affected benchmarks on master:

BenchmarkEvaluationNumericLiteral-8         20000000            79.6 ns/op         1 B/op          1 allocs/op
BenchmarkEvaluationLiteralModifiers-8       10000000           133 ns/op          16 B/op          2 allocs/op
BenchmarkEvaluationParameters-8             10000000           167 ns/op          17 B/op          2 allocs/op
BenchmarkEvaluationParametersModifiers-8     5000000           319 ns/op          40 B/op          4 allocs/op
BenchmarkComplexExpression-8                 3000000           482 ns/op          83 B/op          6 allocs/op
BenchmarkConstantRegexExpression-8           2000000           760 ns/op          48 B/op          7 allocs/op

Post-change:

BenchmarkEvaluationNumericLiteral-8         20000000            61.1 ns/op         0 B/op          0 allocs/op
BenchmarkEvaluationLiteralModifiers-8       20000000           117 ns/op           8 B/op          1 allocs/op
BenchmarkEvaluationParameters-8             10000000           147 ns/op          16 B/op          1 allocs/op
BenchmarkEvaluationParametersModifiers-8     5000000           287 ns/op          32 B/op          3 allocs/op
BenchmarkComplexExpression-8                 3000000           427 ns/op          80 B/op          3 allocs/op
BenchmarkConstantRegexExpression-8           2000000           681 ns/op          48 B/op          6 allocs/op

[Knetic]

Awesome! I love this change. Thanks for submitting it!

[prashantv]

Thanks for the library @Knetic

Just started playing with it today and it's been great!