Rename bdk crate to bdk_wallet

[LLFourn]

All it has in it is a Wallet and the README says:

The bdk crate provides the Wallet type which is a simple, high-level
interface built from the low-level components of [bdk_chain].

It should probably just be called bdk_wallet

[evanlinjin]

Thanks for creating a ticket. Everyone was thinking this for a long while.

[notmandatory]

I put this on the project board, I think we should do it after #1203 and #1101 are merged. But should grab the bdk_wallet account in crates.io ASAP.

[thunderbiscuit]

Feels right to me as well. It makes it explicit that the wallet is one of the crates the bdk project supports without giving it a special status (in that sense the simple bdk name was more confusing than anything given the 6 new crates).