support optional parens in WATF format

[ColinEberhardt]

The following are equivalent, and supported by wat2wasm:

(func $test 
    (i32.const 12)
    (i32.const 0x70000000)
    (i32.store)
  )

(func $test 
    i32.const 12
    i32.const 0x70000000
    i32.store
  )

Currently the WATF parser expects all instructions to be enclosed in parens.