math: implement assignment operators

[andrei8l]

Summary

None

Purpose of change

The math parser enforces an expression split in 1/2/3 strings as a design choice, in order to get neat(-ish) line breaks for long expressions. In practice though, most expressions are short and this design choice leads to a mess of commas and quotation marks.

Describe the solution

Store intermediary function nodes as prototypes and only resolve them when operators/parens/eof/whatever are resolved.
Add assignment operators.
Reduce the eoc-math glue to a glorified string concatenator.

In practice this means that:
Instead of this:

{ "math": [ "MORALE_MULTIPLIER", "=", "u_has_trait('SQUEAMISH') ? -2 : -1" ] },

you can write this:

{ "math": [ "MORALE_MULTIPLIER = u_has_trait('SQUEAMISH') ? -2 : -1" ] },

(it was already possible for comparison operators)

but you can also split it into as many strings as you want for the purpose of clarity or whatever:

      {
        "math": [ 
          "MORALE_MULTIPLIER",
          "=",
          "u_has_trait('SQUEAMISH')",
          "?",
          "-2",
          ":",
          "-1"
        ] 
      },

TODO:

• more test units
• fuzz test: I've been running this fuzz target (with this change) with this corpus extracted from data and test units for ~24 hours
• figure out what broke: my head

Describe alternatives you've considered

N/A

Testing

Test units

Additional context

N/A