Trim some undocumented symbols from asn1.h.

ASN1_ENCODING can be unexported because all types using it are now
hidden. This does mean external uses of <openssl/asn1t.h> can no longer
use ASN1_SEQUENCE_enc, but there do not seem to be any such uses.

ASN1_TLC and ASN1_TEMPLATE typedefs are only necessary for users of
asn1t.h. I'm hopeful we can do away with ASN1_TLC once I get to
reworking tasn_dec.c. ASN1_TEMPLATE is somewhat stuck, though all
references should be hidden behind macros.

ASN1_generate_* appear to only referenced within the library. Remove the
unused one and move the other to x509/internal.h. (asn1_gen.c is
currently in crypto/x509 rather than crypto/asn1, so I put it in
x509/internal.h to match. I'll leave figuring out that file to later.)

Annoyingly, asn1/internal.h now pulls in asn1t.h, but so it goes.

Change-Id: I8b43de3fa9647883103006e27907730d5531fd7d
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50106
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/crypto/x509v3/v3_alt.c b/crypto/x509v3/v3_alt.c
index 0c55816..ce1c6e5 100644
--- a/crypto/x509v3/v3_alt.c
+++ b/crypto/x509v3/v3_alt.c
@@ -64,6 +64,7 @@
 #include <openssl/obj.h>
 #include <openssl/x509v3.h>
 
+#include "../x509/internal.h"
 #include "internal.h"