Groups the two components that make up a signature, and provides a way to encode to DER form, which is
how ECDSA signatures are represented when embedded in other data structures in the Bitcoin protocol. The raw
components can be useful for doing further EC maths on them.
What we get back from the signer are the two components of a signature, r and s. To get a flat byte stream
of the type used by Bitcoin we have to encode them using DER encoding, which is just a way to pack the two
components into a structure.