Support iseval rules

[ReubenJ]

While adding PSB2, @IssaHanou and I have encountered multiple situations where an immediately evaluated expression in the grammar would be useful.

In the benchmark, they add random integers and strings to the grammar quite often, meaning the following would be nice to have working

g_with_eval = @csgrammar begin
    Int = ...
    Int = _(rand(1:100))
end

Which would add a single random integer to the grammar that would be the same throughout the entire synthesis process. Currently, we can do something like:

some_rand = rand(1:100)
g_with_eval = @csgrammar begin
    Int = ...
    Int = some_rand
end

Which is fine, but the first implementation would be cleaner.