Protect important macro arguments with parentheses

Background:
Without properly protecting those macro arguments a call like

     HASH_ADD_KEYPTR(hh, table, int_ptr + offset,
       size_t_end - size_t_begin, e);

yields a different hash computation than

     HASH_ADD_KEYPTR(hh, table, (int_ptr + offset),
       (size_t_end - size_t_begin), e);
Merge pull request #13 from gsauthof/fixmacroparens

Protect important macro arguments with parentheses