applet.interface.i2c_target: add generic I2C target applet. This applet uses clock stretching to handle transactions on the host. Unfortunately this excludes many interesting uses of such an applet.

gateware.i2c: add experimental clock stretching support for target.

gateware.i2c: add a restart strobe.

gateware.i2c: fix stop strobe to follow documented behavior.

applet.interface.i2c_initiator: make repl command actually work.

s/master/initiator/ s/slave/target/ in I2C-related code. I was going to do this for a while, given that even the specification itself has to explain that "master" is the device that is initiating a transaction, etc.

applet.memory.24x: use chunked reads. Currently the I2C applet cannot handle reads of 64K and more, and 64K I2C EEPROMs exist; before this commit, trying to read exactly 64K from one would result in a hang.

support.bitstruct: fix codegen for fields starting with _.

applet.sensor.bmp280: fix reset-on-error code path.

applet.sensor.bmp280: reset the sensor on startup.