Set Memory Limit through API/SDK

[sjudson]

#145 adds support for dynamically setting the memory limit through a macro exposed by nexus-rt.

#146 adds a low-level programmatic API, with an eye longer term towards a stable SDK (#148).

Longer term, it likely makes sense for the API/SDK to also (or even just exclusively) support programmatically adjusting the memory limit.

Doing this likely has a few moving pieces: we ideally want to set the memory limit by just emitting the linker script dynamically, rather than overwriting the stack pointer on start as #145 did. We'd also then need to support dynamic compilation or some sort of rewriting pass over the already compiled NVM assembly (or both), since at present the API only consumes an already compiled program.