Change literal value of s_none symbol in PyrObject

[mossheim]

Fix for #2614.

s_none is needed in the current version of the class and primitive parser as a dummy symbol for "no superclass" or "no name", because other more serious dependency tests are performed using nullptr. However, the use of the symbol literal none caused the interpreter to crash if a method called "none" was added to a SC class, presumably because the parser would attempt to point the new method name to the existing dummy symbol and then run into problems later while dependencies were being constructed.

By changing the literal definition of s_none to __none I believe we solve this issue with minimal effort by keeping the symbol out of the method and primitive namespaces, while preserving the semantic value of the literal.

To test, I modified the BasicOpUGens.sc file to begin:

BasicOpUGen : UGen {
   	var <operator;

	none {
		"this works".postln;
	}
...
}

then built and tested sclang via command line:

*** Welcome to SuperCollider 3.9dev. *** For help type cmd-d.
sc3> a = BasicOpUGen.new
-> a BasicOpUGen
sc3> a.none
this works
-> a BasicOpUGen
sc3> a.perform('__none')
[method not understood error message]

I ran similar tests with none as a class method, classvar, instance var, and both classvar and instance var, all with similar positive results.

[telephon]

I suppose that the double _-prefix (instead of a single one) is needed because single prefix _ is reserved for primitive calls? So maybe you could write

`__` to avoid collisions with possible method and primitive names.

[mossheim]

I suppose that the double _-prefix (instead of a single one) is needed because single prefix _ is reserved for primitive calls?

Technically it's not necessary, since primitives begin with _[A-Z], but this way it's clear that it's part of neither namespace.

[mossheim]

Needed some slight resolution after I merged my other PyrObject.cpp edit. Waiting for travis build to complete for good measure, then I'll merge.