Add in a new core language file.

 This will let us implement common functions seen in other lisps,
 and allow them to be importable, without explicit imports. The goal
 is to keep this as small as we can; we don't want too much magic.

 I've added `take' and `drop' as examples of what we can do.
documented else / finally syntax for try / catch
Merge pull request #202 from paultag/paultag/feature/start-language-lib

Add in a language lib.
Get Hy working on Windows by making readline use conditional.
Merge branch 'master' into pr/213
Style fixes.
Updating NEWS entries.
Move code from hy.readline_helpers into new hy.completer.completion context
manager so we can "wrap" readline completion around REPL interaction.
documentation for some of the builtins
references #18
Style fixes.
Merge branch 'master' into pr/222
Jank @olasd's hack, clean up core.

 The core shall from now on be only for the core language bits. Macro
 bits shall live in hy.macros and in hy.compiler. This cleans up
 garbage.
Don't leave cruft around in sys.modules when an import fails.

Closes: #214, #225

Squashed from a bunch of commits by @olasd
Fix flake error
Adding NEWS entry
Preparing 0.9.9
Include hy files in the release.
0.9.10 release. Bugfix only.
Add rply to dependencies

Use the hylang fork until mainline grows the source position patch.
Add an rply-based lexer