Speed up generated decoder compilation times

[evnu]

This pull request speeds up compilation by extracting common code from field assignments into helper functions, and by preferring local variables for initial assignment instead of assigning struct fields directly. See #299 for the corresponding issue.

Fix #299.

[evnu]

Some benchmark results from running meta_benchmark.sh in https://github.com/evnu/rusty/tree/master/native/benchmark_compile.

~/tools/rusty/native/benchmark_compile(master ✗) ./meta_benchmark.sh

Derive Type	master	299-speed-up-decoder-compilation
NifMap
NifStruct
NifRecord
NifTuple

This looks very promising. I was not yet able to refactor NifRecord and NifTuple to achieve a similar speed up. The problem is that we need to introduce let assignments for the fields, but with tuple structs we do not have a field name available to name the let variable.

[evnu]

I managed to use let for tuple and records as well and squashed it into 2c36dee. Now, compilation is fast for NifTuple and NifRecord as well:

Derive Type	master	299-speed-up-decoder-compilation
NifRecord
NifTuple