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
dump.h: parenthesize negative errnos
remove mrb_str_literal declaration

definition was removed in 677a2ac
mrb_str_cat2: deprecated since 0cedf8f
MRUBY_VERSION should be only numbers separated by dot, like JRUBY_VERSION or Rubinius::VERSION
Merge pull request #1701 from cremno/do-not-use-mrb_str_cat2

mrb_str_cat2: deprecated since 0cedf8f
Merge pull request #1704 from cremno/rm-mrb_str_literal-decl

remove mrb_str_literal declaration
Merge pull request #1703 from cremno/parenthesize-mrbdump-errnos

dump.h: parenthesize negative errnos
Merge pull request #1705 from takahashim/version-number

MRUBY_VERSION should be only numbers separated by dot, like JRUBY_VERSION or Rubinius::VERSION
Merge pull request #1702 from cremno/hash-hash-ext-changes

hash / hash-ext: various small changes