Add hclext.BoundExpr

[wata727]

See also terraform-linters/tflint#1537

This PR introduces a concept of "bound expression". This is part of the extended hcl.Expression provided by hclext package. This expression can bind any cty.Value, and the evaluated value can be bound to the expression.

This is the mechanism needed to support each.key and count.index. When retrieved as hclext.BodyContent, all blocks must have the same raw expression representation, but when evaluated each block must return a different value depending on the context.

We achieve this behavior by binding the expanded context value to the raw expression and returning it as hclext.BodyContent.