Allow => from a UGen into a field/parameter

[heuermh]

A common use case in digital synthesis is to use an oscillator as an LFO for a parameter. The built in UGen oscillators provide some of such functionality via the sync(int) method, allowing input to sync to frequency or to phase.

If however a user would like to use an LFO to modulate the gain of a UGen or to modulate any other parameter (e.g. BlowBotl vibratoFreq or Delay delay) they either need to spork a method at sample rate

Tremolo.ck https://github.com/heuermh/lick/blob/master/Tremolo.ck#L32 https://github.com/heuermh/lick/blob/master/Tremolo.ck#L93

extend Chugen

Trem.ck https://github.com/heuermh/lick/blob/master/Trem.ck#L23 https://github.com/heuermh/lick/blob/master/Trem.ck#L127

or use a higher level construct such as Module

Module.ck https://github.com/heuermh/lick/blob/master/Module.ck

moduleExample.ck https://github.com/heuermh/lick/blob/master/examples/moduleExample.ck

It would be much nicer from a user's point of view if they could do the following

SinOsc sin => dac;
440.0 => sin.freq;

SinOsc lfo;
1.0 => lfo.freq;

// modulate sin osc gain with LFO
lfo => sin.gain;

1::minute => now;

Migrated from https://github.com/spencersalazar/chuck/issues/35