Improve defaults for find-tag.

This implements the following heuristics for the string that
find-dot (M-.) searches for by default:

 1. when point is in an import statement,
    use the name of the imported module.
 2. when point is inside a PTS variable name,
    use that name.
 3. otherwise, provide no default.
Add & demo wrapTypecheckPushUntyped wrapper

Before this wrapper, the example in the README was harder to write:

```
*PTS.Interactive> Right a <- wrapTypecheckPull (Just Instances.uu) [pts|* -> *|] []
*PTS.Interactive> wrapTypecheckPush (Just Instances.uu) [pts|lambda x. x|] a []
```

I also find typecheckPushUntyped's implementation rather elegant.
Improve pts-module-import-at-point.

Also recognize module import with no whitespace between the module name
and the semicolon or end-of-line.
Keep inferred type annotations.

This fixes a bug that lead to spurious "found wildcard during
evaluation" errors. The constructed term with the wildcards replaced by
inferred type annotations was thrown away, and evaluation continued with
the term from the source file.
Extend argument list syntax (see #66).

Argument lists after lambda and Pi keywords now accept multiple names in
one binder even without parentheses.

Accepted before: lambda (x y z) . lambda (a b : Int) . x
Additionally accepted now: lambda x y z . lambda a b : Int . x
Merge pull request #93 from Toxaris/improve-find-tags

Improve defaults for find-tag.
Fix typecheckPush on Const
Merge pull request #96 from Blaisorblade/topic/fix-const-typecheck

Fix #97: typecheckPush succeeds too often on Const
Fix bug in difference printer.

This fixes a long-standing bug about quotation marks showing up in the
difference between types that pts attaches to error messages.
Make pts-* executables conditional (fix #102)

One might later want to change the description or the flag, depending on
what is meant here. But I think this will do.
Add faster variable freshening

Comparing with master:

$ time pts --quiet --instance uu UU/D/count.lpts

real	0m5.665s
user	0m5.574s
sys	0m0.075s
$ time ./pts-master --quiet --instance uu UU/D/count.lpts

real	0m6.063s
user	0m5.969s
sys	0m0.076s

I repeated this more often, and the results look relatively stable.

For testing, I've used
Toxaris/tsr-experiments#636a1f5f9a6c79f8becaa98486f80a3f1c00221f with
`countRebuildIncompl` commented back in.
Speed up operations with insertLookupWithKey