cmd/compile: use less memory for large []byte literal

<pre>[]byte literals take up a lot of memory inside the compiler, because each byte in the
literal is a separate syntax Node and, worse, each byte is represented by a
multiprecision integer constant.

Probably a trick is required during parsing to turn []byte{...} into an actual byte
array holding the constant values + a list of index and value for the non-constant data.</pre>
