Change :vattr= to :cattr= in respond_to? test

fix #1617

class/module body to honor whether value is required or not; reduce a few instructions for normal cases

verify the argument of String#=~

Remove 'mrb_state' field from 'kh_xxx_t' structure.

change behavior of mrb_sym2str

'kh_xxx_t' requires 'mrb_state' to allocate, free, and compute hash value.
But 'mrb_state' should not be held by 'kh_xxx_t' and 'mrb_state' should be
supplied from outside.

add some tests for case statements

fix codegen bug in NODE_CASE

fix TypeError message

Fix typo from expornent to exponent

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>

Update the INSTALL information to be more accurate

Fixed callinfo buffer overflow while calling ensure handlers

Avoid recursive instance variable inspections

Fix mod.constants cannot return the single character constant

Fix ISO no in Array#* and Array#+