Use more higher-order functions
Use more higher-order functions

This commit removes a number of special-purpose helpers and replaces
them with calls to "map", "fold-left", and the like.  The result is less
source code, though at a cost of more indirect calls.

Bootstrap results are neutral, I guess having less code compensates the
indirect calls:

```
Compiling stage1 compiler...
  Time: 4.238 s

Compiling stage2 compiler...
  Time: 4.053 s

Compiling stage3 compiler...
  Time: 4.072 s

Percent change (1 -> 3): -4%
Percent change (2 -> 3): 0%
```

google/schism,google/schism,google/schism