gohcl: Struct Embedding

[cam72cam]

A common pattern that does not currently work for decoding hcl is:

type Base struct {
    Field string `hcl:"my_field"`
}

type Variant struct {
    Base
    Value int `hcl:"my_value"`
}

This is fixed by making some recursive tweaks to schema.go and corresponding changes in decode and encode.

This code is functional for decode, but still requires encode changes as well as test coverage. I may end up refactoring this a bit as I fiddle with encode, but the core functionality is there.

[hashicorp-cla]

All committers have signed the CLA.