Add excludes to manifests

We can check which files are included in the packaged release with
 `cargo package --list `.

Add an `exclude` section to each manifest that excludes `tests/` and
`contrib/`. Not all crates have a `tests/` directory yet but they should
so add the exclude anyway to future proof the crates.

bitcoin/Cargo.toml

@@ -9,7 +9,7 @@ documentation = "https://docs.rs/bitcoin/"
 description = "General purpose library for using and interoperating with Bitcoin and other cryptocurrencies."
 keywords = [ "crypto", "bitcoin" ]
 readme = "README.md"
-exclude = ["tests/"]
+exclude = ["tests", "contrib"]
 edition = "2018"
 
 # Please don't forget to add relevant features to docs.rs below

hashes/Cargo.toml

@@ -10,6 +10,7 @@ documentation = "https://docs.rs/bitcoin_hashes/"
 keywords = [ "crypto", "bitcoin", "hash", "digest" ]
 readme = "README.md"
 edition = "2018"
+exclude = ["tests", "contrib"]
 
 [features]
 default = ["std"]

internals/Cargo.toml

@@ -10,6 +10,7 @@ categories = ["cryptography::cryptocurrencies"]
 keywords = ["internal"]
 readme = "README.md"
 edition = "2018"
+exclude = ["tests", "contrib"]
 
 # Please don't forget to add relevant features to docs.rs below.
 [features]