Change `define`->`def`, `value`->`val`, `function`->`fun`,
and `forward`->`let`.

Also, precedence declaration now supports `same-on-left-as` and
`same-on-right-as`, used to disallow `*` on the right of `/`.

These additions triggered a further revision of static info and
bindings, which has evolved to an even more Turnstile-like
bindirectional protocol at the level of bindings.

Partial reversal of previous design choice. Also, printing is now
implemented.

Also, reorganize macro forms to `macro` and `rule` variants and add
`rhombus/macro` for importing macro and compile-time bindings.

 'keyword' -> ~keyword

 ?  ->  '
 ¿  ->  $
 +$ ->  &

For example, `1+2` immediately produces `3` without an extra line
containing `;`.

The special treatment of keywords as keys does not seem like a good
idea after all.