[FIRParser] unexpectedly accepting of invalid FIRRTL

This started with a module definition at the wrong indentation level-- we currently we accept "FIRRTL" like this:

circuit Oops:
  module Oops:
  input in: UInt
  input out: UInt
  wire w: UInt
  w <- in
  out <- w

Producing an empty module "Oops" and discarding the statements.

Poking at things, it seems we even accept things like:

circuit Oops:
  module Oops:
    input in: UInt<1>
    output out: UInt<1>
    out <- in

  asdf:
    dsfa

asdf
input in: UInt
  input out: UInt
  wire w: UInt
  w <- in
  out <- w
  input in: UInt

Start at the FIRParser entry point and reproduce the malformed FIRRTL examples in the issue. Trace how invalid indentation and stray content are handled; done means malformed input is rejected rather than accepted with statements silently discarded.