This should be analogous to C struct initalizers where all unspecified
fields are initialized to zero. Without mut this reads a bit nicer as
well.
Note that two out of three structs have all members specified, hence
need no zero-initialization of the remainder at all.
They can actually be shared with multiple threads at the same time
safely as all functions requiring an immutable reference are
thread-safe.
OTOH TypeFind can't be shared safely between different threads as not
all implementations of the TypeFind struct are thread-safe.
