extmod/modubinascii: Make code private and module self-contained.

This commit makes all functions and function wrappers in modubinascii.c
STATIC and conditional on the MICROPY_PY_UBINASCII setting, which will
exclude the file from qstr/ compressed-string searching when ubinascii is
not enabled.  The now-unused modubinascii.h header file is also removed.

The cc3200 port is updated accordingly to use this module in its entirety
instead of providing its own top-level definition of ubinascii.

This was originally like this because the cc3200 port has its own ubinascii
module which referenced these methods.  The plan appeared to be that the
API might diverge (e.g. hardware crc), but this should be done similar to
I2C/SPI via a port-specific handler, rather than the port having its own
definition of the module.  Having a centralised module definition also
enforces consistency of the API among ports.

ports/cc3200/mpconfigport.h

@@ -110,7 +110,7 @@
 #define MICROPY_PY_UERRNO_ERRORCODE                 (0)
 #define MICROPY_PY_THREAD                           (1)
 #define MICROPY_PY_THREAD_GIL                       (1)
-#define MICROPY_PY_UBINASCII                        (0)
+#define MICROPY_PY_UBINASCII                        (1)
 #define MICROPY_PY_UCTYPES                          (0)
 #define MICROPY_PY_UZLIB                            (0)
 #define MICROPY_PY_UJSON                            (1)