the new Type-based ASN1 Stream implementation

> From: Robert Withers
>
> With inspiration from VW, I've implemented a new ASN1 Stream and
> associated Type hierarchy.  I briefly explain the new system below.
> This has been published alongside the existing ASN1Value
> implementation, in the Cryptography repository, so that users of
> ASN1Value won't currently be impacted.  However, I would like to
> deprecate the old implementation this week, so this is a call to port
> uses of ASN1Value to ASN1DERStream.  Please contact me if you are a
> user and I will help you port your code.  I will be porting the X509
> Certificate implementation over the next few days, so users of this
> framework should also contact me, as the public api will change.
>
> The new ASN1 framework has a Type hierarchy that supports both basic
> types and user-defined types.
>
> The support for basic types is sufficient to replace the current
> ASN1Value impl.  Below are the replacements.
>
> ASN1Value fromAsnDer: stream.
> becomes
> (ASN1DERStream onStream: stream) decode.
>
> and
>
> anASN1Value encodeAsnDer
> becomes
> ASN1DERStream new encode: anASN1Value.
>
> The user-defined types are stored in an instance of an ASN1Module,
> for each module defined in the image.  For instance, the X509 Module
> is registered as #x509.  To access the forthcoming Certificate type
> definition in the X509 Module (I have to port X509 first), you would
> write:
>
> type := (ASN1Module name: #x509) find: #Certificate.
>
> In defining a type, you can specify a base type with an new name, or
> you can specify sequences and other constructs.  With sequences, you
> can add a mapping class, so the elements of the sequence will be set
> to various ivars.  You can add elements with optional, default or
> explicit and implicit tagging.  Here is a sample of a user-defined
> type, a sequence, mapped to an ASN1TestModel, and a Boolean element
> mapped to the #testSlot1 ivar:
>
> ((ASN1Module name: #test) sequence: #TestSequence)
> add: #testSlot1 type: #ASN1BooleanType;
> mapping: ASN1TestModel;
> yourself.
>
> Now you can encode and decode using the defined Type, and this will
> marshal all values into ivars defined on various mapping classes.
> Here is the corresponding code:
>
> type := (ASN1Module name: #test) find: #TestSequence.
> obj := (ASN1DERStream onBytes: bytes) decodeWithType: type.
>
> and
>
> type := (ASN1Module name: #test) find: #TestSequence.
> bytes := ASN1DERStream new encode: obj withType: type.
>
> Additional examples, especially of the variety of basic types and
> user-defined type specifications, can be found in the CryptoASN1Test,
> once you have loaded the latest Cryptography-ASN1 and Cryptography-
> Tests.
>
> enjoy!
>
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