When disassembling functions for the inspector, we used an internal
text representation before. This CL implements the official text
format like it is understood by the spec interpreter.

Example output:
func $main (param i32) (result i32)
block i32
  get_local 0
  i32.const 2
  i32.lt_u
  if
    i32.const -2
    return
  end
  get_local 0
  call_indirect 0
end

R=rossberg@chromium.org, titzer@chromium.org
BUG=chromium:659715

Review-Url: https://codereview.chromium.org/2520943002
Cr-Commit-Position: refs/heads/master@{#41172}