Fix to allow projects to compile with -Wconversion

Projects that use -Wconversion don't compile due to errors in msgpack header
files. This commit fixes all the errors in msgpack-c for when -Wconversion is
enabled.

Now I can even compile msgpack-c itself with -Wconversion and all my projects:

    CFLAGS=-Wconversion ./configure
		make
Remove implicit type conversions from (un)pack_template.h
Merge branch 'philix-Wconversion'