added set functions for data
minor change

defined bool_t as short int and define true and false constant
Fixed some checks...
Moved waveform generation to synth.c from main

generate_wave() takes sr and note_frqs array to generate output wave

I prefer to leave the srate function and note_frqs in main because i
noticed that it happens something strange if i move in other files...
dunno why... if i move, then the program works well with input from
midi keyboard, but stop to play when i send input trough rosegarden,
even if printing on stdeer out[i] shows that data is written
correctly... so, for now, i'll leave them on main source file, with
process() callback.
Minor change, remove redundant switch
implemented adsr_envelope() function

adsr_envelope function return value of amplitude related to adsr par
Moved note buffer implementation to notes.c
Merge pull request #6 from MicheleSr/develop

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