Style `toString` functionality about penrose

[liangyiliang]

Changes to Library Functions

Some library functions take in strings. We need to make sure those functions still work as intended. Some parameters of these functions require constant strings that are not constructed from numbers (i.e. only ConstStr). While each function can be responsible for checking whether each string is ConstStr, perhaps we need to extend the function caller type system to be able to specify and automatically check this.

One function, TeXify, would need pretty drastic changes. TeXify relies on splitting a string on the _ character. We need to reason about how these types of splitting would work. Implementing custom splitters shouldn't be too difficult though.

[liangyiliang]

For the TeXify function the most important is splitting on the underscore _ character. Observe that _ can only appear within ConstStr, so the only case we need to handle is that, which just entails using plain split string splitting functionality.