To encourage organizing code in a way that helps in understanding, I
have implemented the common idiom of requiring a `main` function. In
tricu and other functional languages, it is usually placed near the top
of the module. The evaluator gracefully handles the situation of passing
multiple files where the intermediary "library" files do not have main functions.
tricu now allows defining terms in any order and will resolve
dependencies to ensure that they're evaluated in the right order.
Undefined terms are detected and throw errors during dependency
ordering.
For now we can't define top-level mutually recursive terms.
Adds support for several special characters in identifiers. Adds a demo
for converting values to source code and another for checking equality.
Updates the existing demo and tests to reflect new names for functions
returning booleans.
