hclwrite: parse object-construct expressions

[bbasata]

Description

Builds an AST for object-construct expressions such as:

a = {
    hat = "derby", (cat) = "calico" }

Given an hclwrite.Expression, an application can read an object value as:

object := expr.AsObjectConsExpr()
value := object.ValueFor("hat")

tokens := value.Expr().BuildTokens(nil)

Retrieval is based on matching a Go string to an HCL identifier. As an implementation choice, there is no way to retrieve the value for the variable expression (cat) at the time of this pull request.

#811 is a speculative pull request that builds on this one and introduces the capability to rewrite objects.

Related Issue

#695 and design notes from Martin in #695 (comment).

How Has This Been Tested?

hclwrite/parser_test.go