fix: tweak encode::Context with future backwards compatibility in mind

Summary

• feat: impl {Encode, Decode} for [T; N]

• feat: impl {Encode, Decode} for Option

• test: test Encode/Decode for integers

• fix: don't allocated in {Encode, Decode} for char

• feat: impl {Encode, Decode} for {HashMap, HashSet, BTreeSet}

• feat: impl {Encode, Decode} for pointer types &, &mut, Box, Rc

• test: be more explicit about what we check in encode tests

• fix: tweak encode::Context with future backwards compatibility in mind

• All fields are private and are accessed via getter methods;
• No Context::new function, as there's no good set of parameters the users are most likely to set. Instead the builder-style methods will be used to set context parameters;
• Context::style renamed to Context::struct_style to avoid confusion;
• EncodeStyle renamed StructStyle to avoid confusion;

• ci: display cpuinfo before running the benchmark

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files