sparse_mmap: remove C code and add SAFETY comments

[romank-msft]

To a certain extent this change "hardcodes"/"emulates" setjmp and longjmp (not their sig counterparts though). These might be tightly intergrated with the system C library making this code brittle. Need to look more into that.

The unit-test do pass on Linux and macOS, but sometimes the Windows guest wouldn't boot in the integration tests (tested only in nested so far). I might be cargo-culting something.