Rename mem.set to mem.fill (WebAssembly#6)

I think it's a better name than `set`, especially considering we may have a `table.set` in the future, which will likely set just one value.

It also follows naming of [Array.prototype.fill](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/fill) which has similar behavior.

proposals/bulk-memory-operations/Overview.md

@@ -156,9 +156,9 @@ new `*.copy` instructions:
 * `mem.copy`: copy from one region of linear memory to another
 * `table.copy`: copy from one region of a table to another
 
-Filling a memory region can be accomplished with `mem.set`:
+Filling a memory region can be accomplished with `mem.fill`:
 
-* `mem.set`: fill a region of linear memory with a given byte value
+* `mem.fill`: fill a region of linear memory with a given byte value
 
 TODO: should we provide `mem.clear` and `table.clear` instead?
 
@@ -239,7 +239,7 @@ The instruction has the signature `[i32 i32 i32] -> []`. The parameters are, in
 - top-1: source address
 - top-0: size of memory region in bytes
 
-### `mem.set` instruction
+### `mem.fill` instruction
 
 Set all bytes in a memory region to a given byte.