Pacify MSVC warnings for numeric.c, proc.c, and symbol.c

b72e94fa used _etext and _edata to distinguish C string literals from heap allocated strings,
but using _edata, global char arrays may be considered as string literals.  to avoid the issue,
we have to use __init_array_start, which might be less portable.  you can still use _edata, by
using MRB_NO_INIT_ARRAY_START.

on platforms with _etext and _edata, mruby can distinguish string literals so that it avoids memory allocation to copy them.
for example, on my Linux box (x86 32bit), memory consumed by mrbtest decreased from 8,168,203 to 8,078,848 (reduced 88KB).

remove unnecessary trailing comma to remove pre C99 declaration error with -Wdeclaration-after-statement

instead of adding sym->name hash table, linear symbol table is added, and reduced name->sym hash table size.

target_class should not be TT_ICLASS in instance_eval(string); ref #1152

fixed. closures scope in eval(string)

Remove duplicated mrb_print_backtrace() declaration

It is declared in mruby.h.

Fix `mrb_get_args` arguments type error found by mruby-clang-plugin.

Wrap README, etc. to 80 columns

change class argument of mrb_const_defined_at from `struct RClass*` to `mrb_value` to make it consistent with mrb_const_defined; ref #2593

Implement Hash[]

update outdated links to the original program.

if you see compiler errors due to this commit, you'd better to use array-modifying
functions, e.g. mrb_ary_set() or mrb_ary_push(), otherwise you might see nasty
GC bugs in the future.  if you are sure what you are doing, replace `RARRAY_PTR(ary)`
by `mrb_ary_ptr(ary)->ptr`. but be warned.

Add inline to str_with_class().

Fix MSVC C++ ABI build. Close #2588.