Add arbitrary precision integer support.

Some functionality is still missing (eg and, or, bit shift), and some
things are buggy (eg subtract).
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index 5b2503f..456ec02 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -14,7 +14,7 @@
 #define MICROPY_ENABLE_LEXER_UNIX   (1)
 #define MICROPY_ENABLE_SOURCE_LINE  (1)
 #define MICROPY_ENABLE_FLOAT        (1)
-#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_LONGLONG)
+#define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_PATH_MAX            (PATH_MAX)
 
 // type definitions for the specific machine