Update Fixnum for 64bits

[higepon]

Currently Mosh's Fixnum assume pointer size == 32 bits. But now 64 bits is majority. We should make it work for both 32 bits and 64 bits.

Basic design

We always use fixedint for Finxum related fucionts in C/C++ world which corresponds to int64_t in 64 bits and int32_t in 32 bits.

Steps

• Update typedef for fixedint.
• Update Fixnum.h
• Build and test both on 32bits and 64 bits.
• Check FASL serialization

[higepon]

https://github.com/higepon/mosh/pull/227