Prevent body-less lambdas from returning true instead of nil.

Fixes #320.
Fix a bug where destructuring was treated as a value.
Update some docs with feedback from uvtc.
Fix make-searcher to save off root opts earlier.
Fix bug in global mangling with env.

Fixes #319.
A couple bug fixes, plus a commented-out test.

* typo correction
* the function to reset the root was returning a function instead of resetting

The test is a problem that has existed since 0.2.0 which only affects
mangled globals; it is not new from self-hosting.
Merge pull request #315 from uvtc/patch-1

tutorial: note on naming identifiers
Update tutorial.md
Merge pull request #318 from uvtc/patch-2

setup.md: tidied up sections
Update tutorial.md
setup.md: tidied up sections

Removed redundant embedded mini-ToC's.
Fixed heading levels so they fall under the correct section.
Heading levels now are "one lower" so they render right in the ToC at the top (I think I have this right).
Also some minor markdown formatting tweaks (ex. list intents)
tutorial: note on naming identifiers
Allow running just one suite in the tests.
Don't use kebab-case for short_src.

As far as I can tell, this is the only place in all of Lua that it
uses snake_case.
[kebab-case] s/assertCompile/assert-compile friend
[kebab-case] s/require-as-include/requireAsInclude
[kebab-case] un-unfix stray opts

they weren't camelCased before

trigger ci rebuild
[kebab-case] camelCase relevant opt{,ion}s fields

- | The changes to the fields in the opts or options tables will need to
  | be reverted; those will cause backwards-incompatibility issues. I
  | think the best thing to do is to keep those as camelCase for the
  | purposes of this patch. After this is merged we can add make further
  | changes to accept those in kebab-case or camelCase if we decide we
  | want to go that way.
  - https://github.com/bakpakin/Fennel/pull/312#issuecomment-663930204
[kebab-case] avoid eval-compiler collision

a patch from @technomancy
- https://p.hagelb.org/0001-Avoid-eval-compiler-collision.patch