Remove constructor for Symbols

[floitschG]

It would be great, if we could just remove the constructor for Symbol.

However, that would mean, that we have to provide a different syntax to construct symbols that contain dots.

Also, the new Symbol constructor is currently used by some packages (most importantly mockito, but also pageloader.

[lrhn]

The literal syntax already allows dots, #a.b . c is equivalent to const Symbol("a.b.c").

We need a way to create setter names and operator names, though.

We could allow them as literals (#a= and #a.b= would not be ambiguous, #[]= and #+ likely not either, but #unary- might be a parsing problem - but probably something the lexer can handle).
Alternatively, we just have constants on Symbol like Symbol.plus, Symbol.integerDivision, Symbol.index and Symbol.unaryMinus and instance setter-conversion (#a).toSetterName() -> #a=.

[rmacnak-google]

FWIW, Smalltallk has short syntax for symbols that are selectors (#unary #+ #keyword:) and long syntax for arbitrary symbols (#'with some spaces').