Change VMMemoryDefinition::current_length to usize

[alexcrichton]

This commit changes the definition of
VMMemoryDefinition::current_length to usize from its previous
definition of u32. This is a pretty impactful change because it also
changes the cranelift semantics of "dynamic" heaps where the bound
global value specifier must now match the pointer type for the platform
rather than the index type for the heap.

The motivation for this change is that the current_length field (or
bound for the heap) is intended to reflect the current size of the heap.
This is bound by usize on the host platform rather than u32 or u64. The previous choice of u32 couldn't represent a 4GB memory
because we couldn't put a number representing 4GB into the
current_length field. By using usize, which reflects the host's
memory allocation, this should better reflect the size of the heap and
allows Wasmtime to support a full 4GB heap for a wasm program (instead
of 4GB minus one page).

This commit also updates the legalization of the heap_addr clif
instruction to appropriately cast the address to the platform's pointer
type, handling bounds checks along the way. The practical impact for
today's targets is that a uextend is happening sooner than it happened
before, but otherwise there is no intended impact of this change. In the
future when 64-bit memories are supported there will likely need to be
fancier logic which handles offsets a bit differently (especially in the
case of a 64-bit memory on a 32-bit host).

The clif filetest changes should show the differences in codegen, and
the Wasmtime changes are largely removing casts here and there.

Closes #3022

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[alexcrichton]

@cfallin if you don't mind, can you confirm all the cranelift fiddly bits look ok?

[cfallin]

Logic all looks correct in the Cranelift portions of this! Two notes below about good comments to add (or maybe extracting a helper in one case) but nothing critical.