Semi-implicit dropping of initialization should be explicited

[rajdakin]

(Based on WebAssembly/stack-switching#118)

The current specification prevents initialization information to be propagated through blocks by simply not having variable lists in the rules:

C |- blocktype : [t1*] -> [t2*]    C, labels[t2*] |- instr* : [t1*] -> [t2*]
----------------------------------------------------------------------------
              C |- block blocktype instr* end : [t1*] -> [t2*]

C |- blocktype : [t1*] -> [t2*]    C, labels[t1*] |- instr* : [t1*] -> [t2*]
---------------------------------------------------------------------------
              C |- loop blocktype instr* end : [t1*] -> [t2*]

I understand the rationale behind this decision, and I agree that this is the simplest way that is also future-compatible. However, someone reading through this specification may not notice this as this is too implicit, and I think this decision should be explicited in a note as this is an important caveat.

The note would not have to be long; it could for example be:

This instruction never initializes locals.

EDIT: Another possibility is to add a note similar to the select instruction, such as:

In future versions of WebAssembly, this instruction may specify locals that become initialized.

On a related note, why are block types valid as instruction types when they cannot describe locals indices? Shouldn't they be valid as function types instead?