Give sub-crates unique names

[ounsworth]

For eventual publication on crates.io, the subcrates are just crates and will need unique names.

The work here is do some searching and figure out which names are free. For example, can we do "bc-sha3", "bc-hkdf", etc? Or just go for the full "bouncycastle-sha3" for full brand power?

For an example, take a look at the prefix convention in the rasn project.

[ounsworth]

I think we've decided to go with bouncycastle for the meta-package, and bouncycastle-sha2, bouncycastle-hkdf, etc for the sub-packages.

Reasons:
A) none of these appear to be currently taken on crates.io
B) Even if one does become taken, https://crates.io/policies makes it pretty clear that they are not a fan of name-squatting, so given that bouncycastle is a well-known crypto library, I think we could argue that any other crate named bouncycastle-<cryptothing> is engaging in impersonation.