Add special-case readline completion logic for first-position

[jpco]

We can't complete %pathsearched binaries, but we can complete some things in "first position". So this adds:

• A way to detect the start of commands when those happen in the middle of a line (like after a ; or & or | or {).
• Completion for syntactic forms at first position -- local let for fn %closure match
• Function completion at first position
• Completion for executable files and directories at absolute paths, but not other files, in first position

Shortcomings:

• Start-of-command detection doesn't handle newlines-inside-() properly; I believe doing that would require a novel (within es) method of retaining previous-line state.
• Paths starting with ~/ or ~username/ don't have filtering for whether they're executable, because I didn't want to bother with tilde expansion.

Possible further improvements for builtin completion:

• variable-assignment completion? (like path =)
• control primitive completion better -- right now you'll get primitive-completion if you already typed $&, but it should actually happen at start-of-command position
• A variable to manually store the list of completable commands?

Of course, programmable completion based on semi-parsed input and something like #178 would be the "real", long-term goal, but this is a useful stop-gap and helps inform how its design would need to work.

A funny outcome of this change: path-cache.es makes it so that running a binary once makes that binary name complete-able from then on.

This code is getting messy... Also, my attitude for readline integration (somewhat unlike my attitude for the rest of the shell) is that these kinds of "80%-working" solutions are okay. If other people want to vocally disagree with that, please do :)