Adds DefinitionRegion and the definition region table: a printer wraps
each nested definition it prints inside its parent, and a caller takes one
out of the parent's print instead of printing it a second time —
separatingDefinitionRegions() for the table, content(ofDefinitionRegion:)
for one region, removingIndentation(levels:) for the parent's indentation.
MachOSwiftSection's marksNestedDefinitions builds on it
(docs/DefinitionRegions.md).