revC: rename some schematic symbols to match Kicad update

applet.i2c_tps6598x: better help text.

applet.program_ice40_flash: new applet.

applet.program_ice40: rename to program_ice40_sram.

applet.spi_flash_25c: fix an issue where --pin-hold 0 would be ignored.

README: add IRC channel *waves* Hi Freenode staff! Yes, we do want to use that channel.

arch.xilinx: add empty __init__ file to please setuptools

applet.jtag_pinout: fix discovery for pin ranges not starting at 0.

gateware: use SB_GB_IO instead of SB_IO+SB_GB to reduce delay. Unfortunately, this takes liberties with the FX2 interface; the data is now captured on falling edge, so instead of one full cycle of data validity there is only a half cycle. However, the FX2 interface was certainly broken before, and it seems less broken to me now, so I am committing this anyway. Ideally, the FX2 interface would register the input on the next rising edge. However, this would add two registers in the input path, and we are already using the INFM1 bit in the FX2. Therefore, adding this register would require a radical rethinking of the entire FX2 interface. See #89.

access.direct.multiplexer: tristate unused pins. Fixes #91.