Rename arith library files

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Use consistent names for import functions, and library files.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename import_all. The idea is to use consistent name for library files.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename numtype.h to num_type.h

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>