Change the SPDX license expression from `MIT` to
`MIT AND Apache-2.0 AND BSD-3-Clause` to reflect all of the licenses
that apply to the crate’s sources.
Add license text for `Apache-2.0` and the `BSD-3-Clause` license from
Go’s `time` module to `LICENSE`. Like `MIT`, both of these licenses
require the license text to be distributed with source and/or binaries.