Using hil as a predicate DSL

For now, *hil* always generate string, with `hil.Parse` and `hil.Eval`.

*hil* uses types, but some magic implicit conversion to string happens.

If you spy here : https://github.com/hashicorp/hil/blob/master/eval.go#L407 `v.Exprs` contains a chain of calls (and their arguments), something like `Call(__builtin_BoolToString, Call(__builtin_IntCompare, Literal(TypeInt, 12), Literal(TypeInt, 2), Literal(TypeInt, 1)))` for `${2 > 1}` or `Call(__builtin_IntToString, Literal(TypeInt, 42))` for `${ 42 }`. The string conversion is in the executable AST tree.

I read all the steps, from scanner to parser to AST to eval, puts a lots of ugly `fmt.Println`, but I can't find where the implicit conversion to string happens.

Can you add some options for using *hil* as a simple script language that return typed result ?

Start with eval.go around the referenced v.Exprs code, then trace the scanner, parser, AST, and eval stages through hil.Parse and hil.Eval. Clarify the design for typed results and options for predicate use, and define the expected behavior before implementation.

Written by the indexing model from the issue text.