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