Coding style: Use enum helpers for Attribute::Kind

[rsmmr]

Attribute::Kind is currently a member of Attribute, with custom tagToKind() and kindToString() methods. For consistency with other enums, we should move Kind to outside the class (namespaced to hilt::attribute::Kind), use the util::enum_:* helpers, and provide overloads for to_string() and from_string().

As an existing example, see the corresponding code for function::CallingConvention in hilti/ast/function.h.

[rsmmr]

I realized we have another issue with the new Kind enum: Spicy-side values now reside inside HILTI headers, which isn't great (things like MaxSize and ParseAt is nothing that HILTI should know about). Not immediately sure how to fix that, it's really like we'd need two enum types.