Schema based validation

[jpogran]

• extraneous block labels - e.g. variable "one" "two" {
• missing block labels - e.g. variable {}
• unknown blocks (top level and within resources)
• missing required attributes
• unknown attributes
• missing required blocks (with set minimum instances of block)
• too many blocks (with set maximum instances of block)
• deprecated attributes/blocks (warning)

[radeksimko]

I pushed a minor change in the last commit to aid readability but otherwise it LGTM at this point. There's more work to do on readability/composition but we can do that as part of another PR.