Improve allocate API

[eudoxia0]

The current API is too easy to misuse:

generic [T: Type]
function allocate(count: Index): Address[T];

To allocate a single object, the client should pass 1, but they might easily pass sizeof(T) instead, which allocates more memory.

We should break this up into:

generic [T: Type]
function allocate(): Address[T];

generic [T: Type]
function allocateBuffer(count: Index): Address[T];

This issue was pointed out in: #310