[spec] Add memory.copy exec text (WebAssembly#106)

Also clean up some `table.copy` text.

* WIP on fixes; checkpoint to avoid working over ssh
* Address comments and other fixes
* Add missing `if n=0` case to `memory.fill` and `memory.init`
* Move `n=0` case to the top in `memory.fill`
* Use `\vconst` to ensure constants are well-formed
* Use 3 spaces instead of 4 in `memory.copy` and `table.copy` to prevent
  double-spaced numeric list
* Add `_t` subscript to '`\vconst`
* Add cnt=1 case
* Make prose match formal

Author: binji