* functions.js: (minor) reworking to clean-up initialization of "externally" defined functions (math, blending, `default`).

* `default` function "front-end" reworked for a bit higher-level control from its "back-end" code (e.g. tree.mixin.eval).
`default` function: added error when used with css-guards. updated tests.
Merge pull request #1757 from seven-phases-max/tree-functions-cleanup

Tree functions cleanup + CSS Guards `default` error.