refactor the huge "main" function into smaller functions + module

[japaric]

what the PR title says. I ran the ELFs from PR #218 and they print the expected output.

This is the most non-trivial change that may have changed semantics

• before we were extracting two RAM regions from the probe-rs registry. The "first one" (returned by the iterator) and the "one that contains the initial stack pointer (initial_sp)". we installed the canary on "the first one" but only if it contained the initial_sp
• now, we only extract one RAM region, the one that contains initial_sp, and install the canary there

I think this still does the same, in practice, but not 100% sure.

[japaric]

bors r=jonas-schievink

[bors[bot]]

Build succeeded:

• test (nightly, macOS-latest)
• test (nightly, ubuntu-latest)
• test (nightly, windows-latest)
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• test (stable, ubuntu-latest)
• test (stable, windows-latest)