Allow an optional vendor name filed for non-standard extension names

[kito-cheng]

I would like to add an optional field for vendor name, that could be used as kind of namespace to prevent naming collision for non-standard extension.

[aswaterman]

I don't like the idea of introducing vendors as a semantically significant concept.

Even within vendors, there will be opcode-space conflicts. So, it will always be better to express extension names fully: e.g., we shouldn't allow Xacme as a shortcut to represent Xacmeanvil + Xacmebomb, because those two extensions might not be disjoint--nor should they be required to be.

Introducing hyphens as a semantically significant character is also a bit of a wart.

cc @kasanovic

[kito-cheng]

My intention is make vendor extension more readable, e.g. Xsifive-cache is more readable than Xsifivecache IMO , not a combo set for vendor, e.g Xsifive = Xsifivecache + Xsifiveclic...

However the underline is already used as separator of extension, so the only way is introduce another character for that.

[aswaterman]

A vendor-name/extension-name separator seems like too specific of a way to spend the hyphen.

A more general option would be to introduce hyphens as intra-name separators that have no semantic meaning: e.g., Xacme-anvil would mean the same thing as Xacmeanvil. This is similar to how underscores worked in a previous version of ISA names. It also has the advantage of being backwards-compatible with existing custom-extension names.

Anyway, we should wait for KA to chime in.