Refactor strings (avoid using dynamically allocated memory)

Refactored strings functions to make them use supplied string
instead of allocating new ones.

Additionally, split String/Int.c into several files to avoid define
mess that was present before.

CMakeLists.txt

@@ -94,7 +94,6 @@ set(BH_SOURCE
     src/Util.c
     src/String/Int.c
     src/String/Float.c
-    src/String/Core.c
 )
 
 set(BH_HEADER
@@ -107,7 +106,6 @@ set(BH_HEADER
     include/BH/Queue.h
     include/BH/Util.h
     include/BH/Thread.h
-    include 
 )
 
 set(BH_SOURCE_DUMMY