Bump version 0.19.0-3 -> 0.19.0-4.0.0

We recently bumped the MSRV, this is a major change and requires a
version bump.

We are using a version format that includes the Bitcoin Core version and
the version for this lib, currently the lib version is a single number.
Instead use typical semvar <Major>.<Minor>.<Patch> for the lib.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "bitcoinconsensus"
-version = "0.19.0-3"
+version = "0.19.0-4.0.0"
 authors = ["Tamas Blummer <[email protected]>"]
 license = "Apache-2.0"
 homepage = "https://github.com/rust-bitcoin/rust-bitcoinconsensus/"