revC: more layout work on port A & misc

Glasgow.lib: Add TCA9517 symbol

revC: I/O buffer B routing

firmware: fix illegal IOx manipulation. Something like: IOx |= 0x02; IOx &= 0x01; doesn't guaranteed to work unless both of those are already outputs, since writing an 1 (or 0) to an input doesn't change the value read back. In practice this results in weird inconsistent bugs due to board-to- board variability. Fixes #77.

gateware.fx2: `async` became a reserved word. Fixes #78.

applet.spi_flash_25c: improve description.

applet.spi_flash_25c: cover API with regression tests.

applet: implement record/replay interface tests.

revC: move FPGA I/O banks to a separate sheet, wire I/O buffers

revC: route LVDS