Blocks should let you write your own first-class functions

Currently, blocks assumes that all function parameters are plan values, and won't let you use a parameter as a function. Obviously, this is a bit disappointing in a language based on Haskell. :(

The issue names no files, tests, or entry points. Start by locating the implementation and tests for blocks and function parameters, then determine how first-class function parameters should be represented and validated; done means blocks accept function arguments and the behavior is covered by tests.

Written by the indexing model from the issue text.