X.509: Add simple ASN.1 grammar compiler

Add a simple ASN.1 grammar compiler.  This produces a bytecode output that can
be fed to a decoder to inform the decoder how to interpret the ASN.1 stream it
is trying to parse.

Action functions can be specified in the grammar by interpolating:

	({ foo })

after a type, for example:

	SubjectPublicKeyInfo ::= SEQUENCE {
		algorithm		AlgorithmIdentifier,
		subjectPublicKey	BIT STRING ({ do_key_data })
		}

The decoder is expected to call these after matching this type and parsing the
contents if it is a constructed type.

The grammar compiler does not currently support the SET type (though it does
support SET OF) as I can't see a good way of tracking which members have been
encountered yet without using up extra stack space.

Currently, the grammar compiler will fail if more than 256 bytes of bytecode
would be produced or more than 256 actions have been specified as it uses
8-bit jump values and action indices to keep space usage down.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Revert "Convert video driver low mem function to new shrink API."

This reverts commit 48d2c494a9c635b018d24315b813ee0d7ec44523.
Fixed new Shrinker API for Video Driver and restored use of drop_cache.