Prepare for 5.0.0 release

[lopopolo]

Release 5.0.0 of boba.

boba is published on crates.io.

Breaking changes

• Remove alloc feature. boba has a required dependency on the alloc crate, but gates the availability of the alloc crate with the alloc feature. boba 4.x did not build when compiling with --no-default-features. Make alloc a non-optional dependency. Remove alloc feature #160

Testing improvements

• Add a test that the impl Display for DecodeError is non-empty. Add a test that the impl Display for DecodeError is non-empty #157

Documentation improvements

• Ensure documentation builds without intradoc link errors when compiling without default features. Remove alloc feature #160
• Avoid hardcoding links to std docs when the std feature is enabled. Prepare for 5.0.0 release #161