lispy.el (lispy--symbolp): new function.

* lispy.el (lispy-down): Update.
  (lispy-up): Update.
  (lispy--sub-slurp-forward): Update to use symbol regex.
  (lispy--sub-slurp-backward): Update to use symbol regex.
  (lispy-slurp): Update to use symbol regex.
  (lispy-barf): Update to use symbol regex.
lispy.el (lispy-ace-subword): improve

* lispy.el (lispy-ace-subword): When only a symbol is marked, and it
  contains e.g. "-" or "/", mark the first part of that symbol.

* lispy-test.el (lispy-ace-subword): Add test.