Split Step.hs (#1665)

Both `execConst` and `stepCESK` are huge case statements.  This refactor puts them in different modules.

Also many supporting functions for implementing commands in `execConst` are moved to their own modules.

Whereas `Step.hs` previously had `2931` lines, the new linecounts are:

| File | lines |
| --- | --- |
| `Step.hs` | 848 |
| `Arithmetic.hs` | 124 |
| `Const.hs` | 1687 |
| `Command.hs` | 417 |

The only tricky thing was that `execConst` is mutually recursive with `runCESK`.  So to get them into different modules, I pass a wrapper of `runCESK` as an argument to `execConst`.

scripts/benchmark-against-parent.sh

@@ -0,0 +1,22 @@
+#!/bin/bash -xe
+
+# Requires that the working tree be clean.
+
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+cd $SCRIPT_DIR/..
+
+if git diff --quiet --exit-code
+then
+   echo "Working tree is clean. Starting benchmarks..."
+else
+   echo "Working tree is dirty! Quitting."
+   exit 1
+fi
+
+BASELINE_OUTPUT=baseline.csv
+
+git checkout HEAD~
+stack bench --benchmark-arguments "--csv $BASELINE_OUTPUT --color always"
+
+git switch -
+stack bench --benchmark-arguments "--baseline $BASELINE_OUTPUT --fail-if-slower 3 --color always"