The current Sequence interface is verbose, and we can get a better
experience, and simpler code, by using a SequenceBuilder. The commit
adds and tests SequenceBuilder, which is intended to replace the
public interface of Sequence.

The commit marks Sequence members as private to the crate. The commit
updates all tests and documentation to use SequenceBuilder.

This commit removes the API that generates the configuration block
as an array of magic numbers. Instead, it gives users a way to define
configuration blocks at compile time. The entire interface uses const
methods and functions to achieve the compile-time interface.

TODO: fix all the docs, add more tests.

Since this crate is now compiled into the embedded system -- not just
something that runs at build time -- the crate is now no_std.

This commit was tested in the teensy4-fcb crate. It works as
expected.

Simplify the name, since it's now in the flexspi module.

Use processor family identifiers as feature flags.

See the CHANGELOG for a migration guide.