Revert "Prefer vector to manual linked list for compositions."

Breaks type inference.

This reverts commit 79b1266ed812702f170a945bae49d1688296872f.
Prefer vector to manual linked list for compositions.
Begin rewrite.
Typecheck and interpret simple words.
WIP
Typecheck words.
Typecheck and interpret 'empty' and 'if'.
Differentiate scalar and row types.
Typecheck and interpret 'apply'.
Unsafe vector operations.
Fix vector type 'Show' instance.
Kill dead code.
Fix whitespace.
Typecheck and interpret 'fun'.
Typecheck and interpret 'compose'.
Fix style.
Factor repetition.
Reorganize sources and add testing stub.
Test comments.
Test single-character tokens.