new engine: nanobeep2

nanobeep2 is a tiny sound engine for the ZX Spectrum beeper. Unlike the original
nanobeep, the design goal was not to make the player as small as possible, but
to cram in as much features as possible in less than 100 bytes. 

In it's most basic form, the player takes up 64 bytes of memory. A range of 
additional features can be activated via assembler switches, increasing the size
of the player up to a maximum of 99 bytes.

Core (minimal) player features:

- 2 square wave channels
- global 8-bit tempo resolution
- 8-bit note dividers (~4 octaves, lowest notes may be detuned)
- limited keyboard checking (checks only Space, A, L, Q)

Additional features:

- border masking
- full keyboard checking
- PWM sweep sound
- click drum
- per-pattern tempo setting
- increased note range (6 octaves)