Bump version to 0.10.0-alpha

In preparation for doing an `alpha` release of the new `primitives`
crate bump the version to `0.10.0-alpha` and add a changelog entry.

ref: https://doc.rust-lang.org/cargo/reference/resolver.html#pre-releases

CHANGELOG.md

@@ -1,6 +1,8 @@
-# unreleased (0.10.0)
+# 0.10.0-alpha
 
-- [Implement `std::error::Error::source` for error type](https://github.com/rust-bitcoin/rust-bech32/pull/72)
+This release introduces a new `primitives` module that is basically a new implementation of the
+whole crate. We have not yet settled on the usage of the new API in `lib.rs`, hence the `alpha`
+release.
 
 # 0.9.1
 
@@ -10,4 +12,4 @@
 # 0.9.0
 
 - [Enable edition 2018](https://github.com/rust-bitcoin/rust-bech32/pull/57) bumping MSRV to 1.41.1
-- [Implement `From<u5> for u8`](https://github.com/rust-bitcoin/rust-bech32/pull/58)
+- [Implement `From<u5> for u8`](https://github.com/rust-bitcoin/rust-bech32/pull/58)

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "bech32"
-version = "0.9.1"
+version = "0.10.0-alpha"
 authors = ["Clark Moody", "Andrew Poelstra", "Tobin Harding", "The rust-bitcoin developers"]
 repository = "https://github.com/rust-bitcoin/rust-bech32"
 documentation = "https://docs.rs/bech32/"