Add "expr" type as a schema type

[wata727]

Fixes #165

This PR adds the expr type as a schema type. The expr type differs from other types in that they do not immediately evaluate a value but instead return an expression structure. For example:

variable "instance_type" {
  default = "t2.micro"
}

resource "aws_instance" "main" {
  instance_type = var.instance_type
}

string type

terraform.resources("aws_instance", {"instance_type": "string"}, {})

[
  {
    "type": "aws_instance",
    "name": "main",
    "config": {
      "instance_type": {
        "value": "t2.micro",
        "unknown": false,
        "sensitive": false,
        "ephemeral": false,
        "range": {...}
      }
    },
    "decl_range": {...}
  }
]

expr type

terraform.resources("aws_instance", {"instance_type": "expr"}, {})

[
  {
    "type": "aws_instance",
    "name": "main",
    "config": {
      "instance_type": {
        "value": "var.instance_type",
        "range": {...}
      }
    },
    "decl_range": {...}
  }
]

From the above, you can see that the expression representation (var.instance_type) is returned, not the result of evaluating var.instance_type (t2.micro). Note that attributes such as unknown are undefined because the expression has not been evaluated.

This is useful for writing policies over expression structures. For example, the expr type is the only way to handle meta-arguments such as ignore_changes that cannot be evaluated in the normal way.

The expression obtained by the expr type can be passed to the introduced hcl.expr_list, hcl.expr_map, and hcl.expr_call functions in this PR to perform static analysis of the expression structure. These functions are wrappers around HCL static analysis functions. For example, you can use hcl.expr_list to check for references in ignore_changes:

resource "aws_instance" "main" {
  lifecycle {
    ignore_changes = [key_name, tags]
  }
}

resources := terraform.resources("*", {"lifecycle": {"ignore_changes": "expr"}}, {"expand_mode": "none"})
ignore_changes := resources[i].config.lifecycle[j].config.ignore_changes

# Does "ignore_changes" have "tags"?
some k
hcl.expr_list(ignore_changes)[k].value == "tags"

We have deliberately not introduced functions for static traversal. Perhaps you will need an API like lang.ReferencesInExpr, but it is not yet clear what kind of API you will need, so we will consider adding it upon request.