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

GitHub: fix #2902, #2917

The current implementation traverses stack to retrieve backtrace. But
stack will be changed when some operations are occurred. It means that
backtrace may be broken after some operations.

This change (1) saves the minimum information to retrieve backtrace when
exception is raised and (2) restores backtrace from the minimum
information when backtrace is needed. It reduces overhead for creating
backtrace Ruby objects.

The space for the minimum information is reused by multiple
exceptions. So memory allocation isn't occurred for each exception.