Implementation for primitives about strings.

[Milky2018]

Main contributions:

• Implement Pstringequal and corresponding tests.

Tests and output formats:

• Test file support for g++ and diff output.
• When writing SSA to files, adding type infos.

Bugfixes:

• Bugfix: in SSA generation for switch_constr
• Bugfix: operand types correction for SSA: SLTI, SRAI.

Work in progress:

• Add a function to revert the optimized_option type to its representative type.

[mengzhuo]

This PR conflicts against main, could you resolve it?

[Milky2018]

It has some merge issues. I will raise another PR after resolving.