Add operations for packing and unpacking two i32s into a i64

[codefromthecrypt]

ABIs sometimes target 1.0 which doesn't support multiple returns, and employs a trick to pack two results into one. When writing tests in wat, it is helpful to be able to use shift operations vs doing these by hand.

ex. this is manual

  (global $eof i64 (i64.const 4294967296)) ;; `1<<32|0`

Preferably

  (global $eof i64 (i64.shl (i64.const 1) (i64.const 32)))

To fully implement both ways (ex unpacking) implies #4 i64.extend_i32_u i32.wrap_i64 i64.shr_u