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. :(

该 issue 未指定任何文件、测试或入口点。首先定位 blocks 和 function parameters 的实现与测试，然后确定 first-class function parameters 应如何表示和验证；当 blocks 接受 function arguments 且该行为有测试覆盖时，即视为完成。

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