Add set of new core functions

Add set of new core functions to the stdlib.

Moved the auto-import code from compile_expression to
HySymbol so that "even?' in this style expression will
be found and imported.

(list (filter even? [1 2 3 4 5]))

The core functions are documented in 2 sections, one
for basic functions like (even?..) and (nth ...) and
one for all the sequence functions.

Update: This removes all the caching decorators, misnamed as
'lazy-seq' from the core. All sequence methods now just use
yield to return a generator, so they are Python-lazy

Further refinements of core functions

Cleaned up the docs to use 'iterator' instead of 'generator'

Fixed drop to just return the iterator instead of an extra
yield loop. But also added a test to catch dropping too
many.