Stablize variable typing and support integer truncation and sign extension

[ChAoSUnItY]

This is a attempt to support variable typing for all local variables (including synthesized ones) so that integer truncation and sign extension could be implemented, which targets to fix #166.

Notice that there's still some works including type derivation from operators and code cleanup.

About sign extension in rv32

It seems that package qemu-user is currently outdated (6.2.0) so Zbb extension is not supported, which results in the implementation of sign extension in rv32 currently using straightforward way to implement, that is bitwise shifting.

Summary by Bito

This pull request significantly enhances the variable typing system by introducing support for integer truncation and sign extension, improving local variable management, and optimizing data structures. It addresses existing variable handling issues and lays the groundwork for future enhancements in type derivation and memory management.

Unit tests added: True

Estimated effort to review (1-5, lower is better): 2

[ChAoSUnItY]

Regards to the issue of sign extension on rv32 backend, should we introduce a compiler parameter -Zbb to allow user to choose whether generate sign extension instruction with sext.b (or sext.h in the future once short type is implemented) or with bitwise shifts?

[jserv]

Regards to the issue of sign extension on rv32 backend, should we introduce a compiler parameter -Zbb to allow user to choose whether generate sign extension instruction with sext.b (or sext.h in the future once short type is implemented) or with bitwise shifts?

It is a good idea. However, we then need to an effective way to select the extensions for RISC-V backend.

[jserv]

Let's rename it to fatal. Such helper functions should be always having shorter names.