Support local variables with frame pointers

[michaeldiamant]

Problem

Local variables are implemented using scratch slots because it's been impractical to manage stack-based allocations in PyTeal. With the introduction of frame pointer opcodes (algorand/go-algorand#4319), there's less friction.

The story requests defining + implementing a local variable scheme based on stack allocated values. It's understood that the API must be fleshed out during development.

Solution

Dependencies

Urgency