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]))

This introduces the lazy-seq decorator to wrap around
generator functions and allow reuse. The current name
is there to mimic Clojure, but it could also be called
"cached-generator" to more correctly describe the
underlying behavior.

The core functions are documented in 2 sections, one
for basic functions like (even?..) and (nth ...) and
one for all the sequence functions that use lazy-seq.

The documentation for the Sequence section includes a
description of the behavior of this decorator.