Clean up source text representation in the AST

[chalcolith]

Currently source text fragments are stored in semantic data for a limited set of nodes. When reconstructing a source file, it would be easiest if strings were stored on child Span nodes, and the semantic data pointed to those.

For example, eohippus-fmt currently uses the Loc info from the parse tree to reconstruct leaf node strings; this works because we always read directly from disk.

However, if we're reading from a cached parse tree, the Locs don't exist any more, so we couldn't do this for formatting in the lsp, say.

So we should store strings on the leaf Span nodes and have data fields point to them instead of storing the strings directly.