* Remove uses of `car` and `cdr` in /hy
* Remove uses of `car` and `cdr` in quote tests
* Remove `car` and `cdr` in favor of `first` and `rest`
I beefed up the documentation and tests for `first` and `rest` while I was at it.
I defined `car` and `cdr` in native_tests.cons so the tests read a bit more naturally.
* with-decorator: Allow a `setv` form as the form to be decorated
This feature is of dubious value by itself, but it's necessary to allow `defn` to create a lambda instead of a `def`.
* Make `fn` work the same as `lambda`
That is, allow it to generate a `lambda` instead of a `def` statement if the function body is just an expression.
I've removed two uses of with_decorator in hy.compiler because they'd require adding another case to HyASTCompiler.compile_decorate_expression and they have no ultimate effect, anyway.
In a few tests, I've added a meaningless statement in `fn` bodies to force generation of a `def`.
I've removed `test_fn_compiler_empty_function` rather than rewrite it because it seems like a pain to maintain and not very useful.
* Remove `lambda`, now that `fn` does the same thing
