Overly simple MIDI synth

This is a bare-bones monophonic MIDI synthesizer that uses the pitch and velocity information from incoming MIDI note messages to control the frequency and amplitude of a sawtooth waveform. 

This patch uses the MIDI key number (pitch) data byte to set the frequency of a sawtooth oscillator, and it uses the velocity data byte to set the amplitude of the oscillator. Note-on velocity values in the range 1-127 are mapped to a range of decibels from -60 to 0, and those decibel values are then converted to linear amplitude values to scale the amplitude of the oscillator.

Possible improvements to this synthesizer include:
- handling overlapping or simultaneous incoming MIDI note messages by producing polyphonic synthesis (multiple simultaneous tones in response to multiple note messages).
- an ADSR amplitude envelope to control the loudness better
- implementation of MIDI pitchbend messages to alter the frequency commensurately
- vibrato and/or tremolo effect by means of a low-frequency oscillator (LFO), with the modulation wheel controlling the depth of the modulation effect
- portamento to glide from one frequency to another
- envelope to control the timbre of the instrument so that it changes character over the course of the note, probably dependent on the MIDI note velocity
- etc.