Make SyntaxBuildableType hashable

[fwcd]

This is a small change that implements __hash__ for SyntaxBuildableType on the Python-side, thereby making it possible to use SyntaxBuildableType in sets and dictionaries as keys.

Since the type already implements a fieldwise __eq__, having a corresponding __hash__ feels like a natural extension.

Note that the Swift version of this type (in SwiftSyntaxBuilderGeneration) is also Hashable.

[fwcd]

@swift-ci please test