* license: use SPDX identifiers for src/ headers
Replace the full ~20-line MIT license boilerplate on every src/ file with a
two-line SPDX tag (SPDX-FileCopyrightText + SPDX-License-Identifier), following
the REUSE convention used by CircuitPython and the Linux kernel. Removes ~3500
lines of duplicated boilerplate.
