remove lcsymtacs (github issue #666)

The lcsymtacs structure is regarded superseded as it plainly is
a shorthand for opening the following modules:
Abbrev HolKernel boolLib Tactic Tactical BasicProvers simpLib
Rewrite bossLib Thm_cont

Remove some duplicates of the theorem TRUTH.

These are:
• INFINITE_DEF and IN_LIST_TO_SET (replaced with overloadings)
• IN_APP_applied and IN_ABS_applied (inadvertently generated)

Also, made changes to try to avoid new definitions in listTheory accidentally having a "_DEF" suffix instead of "_def". Old naming inconsistencies still remain in listTheory (e.g. EXISTS_DEF, FIND_def and FLAT).

These changes will break a few proofs but patches should be trivial (i.e. removing references to these TRUTH theorems and/ or renaming "_DEF" to "_def").

Make the zipper example look a little prettier.

- Use Datatype instead of Hol_datatype
- Use [simp] annotations instead of export_rewrites
- Overload zapply to <*>

Prove that the zipper type is what Haskell folk would call "applicative".

A new example demonstrating the zipper datatype (over lists)

Partly inspired by a challenge from Tony Morris