Spec for Memory object's constructor does not check maximum >= initial

[lars-t-hansen]

The descriptor passed to WebAssembly.Memory's constructor has two fields, initial and maximum, where maximum is optional. If maximum is present then there needs to be a check that it is no smaller than than initial, but there's no such check. WebAssembly.Table's constructor does have such a check.

EDIT: Fixed ">" to ">=", though subject line and @binji's PR were both correct, so editorial only.

[lukewagner]

Good spot! It seems like ideally this validation of the limits would happen using the core spec's validation rule for memory types and table types. Maybe the simplest place for this might be in the core spec alloc_mem and alloc_table functions, so that the wasm core spec is preserving its own validation invariants? WDYT @rossberg?

[lukewagner]

Oops, I hadn't reached #877 yet in my bugmail. Either approach works, I suppose.

[rossberg]

@lukewagner, yes it would make sense to add these as explicit pre/side conditions. I'll do that.

[littledan]

Closing per #877