Support rtt.{canon,sub} in constant expressions

[binji]

Also allow multiple instructions when GC feature is enabled. It should
only be used by rtt.sub, since all other instructions have no
operands.

[binji]

Fixes #12.

[binji]

cc @jakobkummerow

[binji]

@sbc100 BTW, the diff looks complicated, but the main change here is that a global initializer can be multiple instructions, so it occurs in a loop. This already works with other instruction lists (like a function body), so I reuse regular instruction validation for most instructions. global.get is weird, since if you use it inside of a global initializer, then only imported globals can be referenced.

[sbc100]

Is what you call ConstantExpression here what I normally think of an initializer expression? Why choose a different term here?

[binji]

ConstantExpression is what the spec calls it: https://webassembly.github.io/spec/core/valid/instructions.html#constant-expressions

They're used for global initializers but also for data/element segments.