Handle shared/frozen strings in IO#sysread

See https://github.com/mruby/mruby/commit/7edbe428caca6814841195a3f2720745cf04353e

Fix for IO#read(n) with n > IO::BUF_SIZE

Support outbuf argument for IO#read

Reseek when write

To accurate `pos`

pos shouldn't cache because it's not controllable

IO#sysread Check for readable

Should raise SyscallError on IO#syswrite

Fix some spec for IO#sysread

instead of IOError

Make backquote work with the latest mruby. Fixed #75

run mrbtest outside of source tree.

Because IO class, which is the superclass of File class, includes it.

Current implementation of Enumerable#hash calls #each to collect
hash values of enumerated objects (then calculates the hash value
of the Enumerable object).  But IO#each is a method to read lines.
It is not expected that IO#hash reads all lines from the IO object.
Or even worse, program waits for the IO object to be readable if
it is a socket or something cannot be read immediately.

Add pointer casting

Add File.readlink

Add File.chmod

Add explicit cast from void* to struct mrb_io*

For compatibility with C++.