Added 16x2 character LCD display init code.
Interrupt-driven I2S.
More stable but still trivial memory dumping over HTTP.
Separate memory dump CGI code to its own CGI handler class. Add another
indirection layer for dispatching requests to handler implementations.
Stub implementation for HTTP parameter handlers.
Somewhat kludged frequency parameter control.
Unfinished but working frequency and level control with atomic parameter
update delivery from M0 to M4 core and to I2S generator interrupt.