Move backward compatibility aliases to their own header

Move backward compatibility aliases to a separate header. Reserve
crypto_extra.h for implementation-specific extensions that we intend
to keep supporting.

This is better documentation for users. New users should simply ignore
backward compatibility aliases, and old users can look at
crypto_compat.h to see what is deprecated without bothering about new
features appearing in crypto_extra.h.

This facilitates maintenance because scripts such as
generate_psa_constants that want to ignore backward compability
aliases can simply exclude crypto_compat.h from their parsing.

tests/scripts/test_psa_constant_names.py

@@ -206,12 +206,6 @@ class Inputs:
         # Auxiliary macro whose name doesn't fit the usual patterns for
         # auxiliary macros.
         'PSA_ALG_AEAD_WITH_DEFAULT_TAG_LENGTH_CASE',
-        # Deprecated aliases.
-        'PSA_ERROR_UNKNOWN_ERROR',
-        'PSA_ERROR_OCCUPIED_SLOT',
-        'PSA_ERROR_EMPTY_SLOT',
-        'PSA_ERROR_INSUFFICIENT_CAPACITY',
-        'PSA_ERROR_TAMPERING_DETECTED',
     ])
     def parse_header_line(self, line):
         """Parse a C header line, looking for "#define PSA_xxx"."""