Shift type opcodes

[rossberg]

With the recent decision for a 3-pronged type hierarchy, the GC proposal needs a few more type opcodes. I suggest leaving some space and moving the string types to opcodes 0x60 and below.

[jakobkummerow]

FWIW the immediate need for this has been obviated by WebAssembly/gc@902830d.

(We might shift the opcodes regardless; I don't feel strongly about that either way.)