hash / hash-ext: various small changes

src/hash.c:
 - mrb_hash_(aget|aset|dup|delete): internal linkage
 - remove documentation of methods which are not implemented (in here)
 - remove #assoc; unused, not in ISO spec
 - remove #rassoc: same, implementation is also wrong

hash-ext mrbgem:
 - remove header "mruby/khash.h"
 - remove mrb_hash_values_at (move code into hash_values_at, i: long -> int)
 - less whitespace in gem_init function
Merge pull request #1702 from cremno/hash-hash-ext-changes

hash / hash-ext: various small changes