Assume <stdint.h> and fixed-width types

Make unconditional use of <stdint.h> and fixed-width types such as
uint32_t.  k5-plugin.h doesn't use any special integer types, so
remove the conditional include block there.  Nothing uses
INT64_FMT/UINT64_FMT, so leave those out of k5-platform.h for now.
Get rid of builtin AES uitypes.h

Remove uitypes.h and just include stdint.h; all we need from it is
uint{8,16,32}_t.
Stop generating gssapi_krb5.h

We started generating gssapi_krb5.h from gssapi_krb5.hin when we
needed to use a 64-bit type for lucid contexts.  Since we can now
assume a standard name for 64-bit types, we can stop generating the
header.
Eliminate internal fixed-width type wrappers

Directly use stdint.h names for integer types in preference to the
various internal names we have made up for them.