SPI refactor.

Still a polling driver, but the libmaple proper interface exposes
enough that users enable the various interrupts and define their own
IRQ handlers if they feel like it.

Wirish HardwareSPI interface was largely redone; it's more like the
Arduino implementation now, although there are some differences when I
didn't like their API.  The old methods are still there, but are
deprecated and slated for deletion in 0.1.0.

New board-specific values: BOARD_NR_SPI, BOARD_SPIx_NSS_PIN,
BOARD_SPIx_MOSI_PIN, BOARD_SPIx_MISO_PIN, and BOARD_SPIx_SCK_PIN, for
x from 1 to BOARD_NR_SPI.

Documentation was updated appropriately.

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