Efficiently adding to an existing SyntaxSet

[9999years]

In Zola, there's a set of builtin syntax definitions and support for loading your own from a directory. To avoid re-linking all the contexts in the builtin syntax definitions, these two SyntaxSets are kept separate.

However, this means that if you write a new syntax definition, you can't reference or embed any syntax from builtin set (getzola/zola#1678). The naïve approach of loading the serialized dump of the SyntaxSet, calling into_builder(), adding the new syntaxes, and build()ing it again causes an 8x slowdown (getzola/zola#2738).

It would be really nice if it were possible to efficiently add to an existing syntax set.