This PR includes the following changes:

1. Add a missing case in the handling of assignments in the interpreter backend, where the LHS is a constant.
2. Allow texts in the term language in SpecTec compiler.
3. Add inverse and builtin hints to spectec that are used by the interpreter backend.
4. Implement all the integer numeric functions as spectec functions, in lieu of their hardcoded definitions in the interpreter.
5. Add several generic auxiliary functions (mostly on sequences) for the numeric operations.
6. Add prose gen tests.

I leave several types of things as comments in the spectec files, instead of removing them, primarily in section 3.1. They are:

1. Failed attempts of some function definitions, due to currently limitations in the implementation.
2. The original definition of inverse functions (those that calls the inverse in the if side condition), because they state the mathematical relation with its inverse, in particular which argument becomes the output, and this is not reflected with the hint.
3. Other misc things that are noteworthy.