Fix passing all zero string to Kernel#Integer

Update README to mention ruby version management tools

Fix build error in ruby 1.8.

Apply change of #2978 to :clang, :gcc, :visualcpp toolchains

Fix class variable reference in module

Fix #3079

Fix segfault on mrb_exc_backtrace.

The code to iterate over backtrace locations was changed in #3065, but
unfortunately output_backtrace was not correctly updated to forward the
callback.

printf precision parameter must be 'int' type

There is a problem when MRB_INT64 is enabled.

symname_p support `!~`

provide macro to ease using printf mrb_int

that means String#index matches first byte of a multi-byte character. this behavior is different
from CRuby, but a compromise for mruby which does not have encoding stuffs.

mruby-fiber: fiber_switch() to use nesting VM when it's called from C API or mrb_funcall(); close #3056

use _setjmp/_longjmp on OS X

Fiber.yield cannot be called from #initialize which is called by
mrb_funcall().  It is mruby limitation.

Use memchr for performance

```ruby
s = "b"
str = ("a" * 100 + s)

t = Time.now
str.index(s)
puts Time.now - t
```

before => 0.000788
after  => 0.000508

---

```ruby
s = "b"
str = ("a" * 100 * 1024 * 1024 + s)

t = Time.now
str.index(s)
puts Time.now - t
```

before => 0.225474
after  => 0.008658

Support backtrace after method calls