Add missing DUK_SINGLE_FILE

This define was missing from the single file header build, which had the
impact of Duktape using incorrect symbol visibility macros.  When this is
fixed, a lot of warnings are generated from unused static functions etc.
This fallout is fixed by follow-up commits.
Add sizes to builtin array declarations

Fixes some compile warnings.
Remove (#if 0) unused functions

These were revealed after fixing DUK_SINGLE_FILE, gcc will complain about
these internal functions being declared, defined, but not used.  There were
also a few declared functions that did not exist.  Compiled binary size was
reduced by a few kilobytes!

Unused functions must be commented out carefully: some functions might be
used with certain feature options but not by the defaults.  These removals
were verified with grep.

There are no call sites for DUK_DEBUG_DUMP_HEAP() so all the functions
in duk_debug_heap.c are unused, even with a debug build.

Out of the checked allocation macros, only DUK_REALLOC_INDIRECT_CHECKED() is
actually in use at the moment.  This commit just comments the unused stuff
out, but it might be better to just remove the unused stuff entirely.
Remove unused (and incomplete) file
Leave removed file out of dist
Don't use strptime() in Cygwin

There's a compile warning of strptime() not being declared.  It should be
declared by <time.h> but is maybe missing a macro.  There are apparently
some limitations in strptime() too, so disable its use for now, as local
time parsing is not a mandatory feature.
Unref fix for Win32 build
Merge branch 'fix-single-file-visibility'

Add DUK_SINGLE_FILE which was missing.  This fix caused a lot of warnings
about unused static functions, so this fallout is also fixed by this merge.

Cygwin compile warning is also fixed by not using strptime() on Cygwin.