sub-tree references and tidiness of macroproc package

[errordeveloper]

As (*BranchLocator).value is a pointer to a subtree, it tracks stale subtrees. We can find other ways to fix this, perhaps rewriting the package in a way that treats the tree as an immutable entity may be a little hard to do in Go (due to the lack of read-only constructs), but we may be able to call modifiers on (*BranchLocator).value instead of (*Converter).tree, and reducing sub-tree reference count would probably help too.

What has been done in #37 works okay (for the MVP), but we should certainly revisit how it works.