Remove unstable hex dep from public API of primitives

[tcharding]

Adds a dep on hex v1.0.0 and uses it everywhere required so that hex v0.3.0 does not appear in the public API.

Note in the serde stuff we use the unstable hex crate still even though we are decoding because we need access to the inners of the errors. This is the exact reason we added that functionality to hex and then during the 1.0.0 saga it was not included.

With this applied we can see from grepping the API text files that only the v1.0.0 error types are in the public API.

gg '.*hex.*' | grep -v to_hex
all-features.txt:1157:pub bitcoin_primitives::transaction::ParseOutPointError::Txid(hex_conservative::error::DecodeFixedLengthBytesError)
all-features.txt:1239:pub extern crate bitcoin_primitives::hex
all-features.txt:1798:pub fn bitcoin_primitives::witness::Witness::from_hex<I, T>(iter: I) -> core::result::Result<Self, hex_conservative::error::DecodeVariableLengthBytesError> where I: core::iter::traits::collect::IntoIterator<Item = T>, T: core::convert::AsRef<str>
all-features.txt:1922:pub type bitcoin_primitives::BlockHash::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1923:pub type bitcoin_primitives::Ntxid::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1934:pub type bitcoin_primitives::TxMerkleNode::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1935:pub type bitcoin_primitives::Txid::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1936:pub type bitcoin_primitives::WitnessCommitment::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1937:pub type bitcoin_primitives::WitnessMerkleNode::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1940:pub type bitcoin_primitives::Wtxid::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1965:pub type bitcoin_primitives::script::ScriptHash::Err = hex_conservative::error::DecodeFixedLengthBytesError
all-features.txt:1975:pub type bitcoin_primitives::script::WScriptHash::Err = hex_conservative::error::DecodeFixedLengthBytesError

Fix: #5211

[nyonson]

Is there a technical reason to use the extern crate syntax over pub use hex_stable as hex;?

[apoelstra]

No, it's just about how they get categorized in the docs.rs output. (I guess that's sorta a technical reason.)

[apoelstra]

In the OP you meant to link to #4808 which is merged now.

[apoelstra]

Nice. Good call naming the old one hex_unstable so we'd see immediately if it showed up in the API files.

[nyonson]

ACK b7f7782

[apoelstra]

ACK b7f7782; successfully ran local tests