Adopt Newspeak Setter Sends instead of Assignment Syntax

[smarr]

This PR solves #7.

To be Newspeak spec compliant, we need to move from the classic var := 42 assignment syntax to the Newspeak setter send syntax where we got var:: 42 instead.
The main motivation for the change is compliance and the ability to load Newspeak code at some point directly.

The changes in the parser and lexer remove the previous use of an Assign symbol, and instead, following the Newspeak spec, lex a SetterKeyword. This makes sure that there can't be whitespace between the two colons.

SOMns Code Transformation

To adapt existing code, I used the following:

find . -name '*.som' -exec sed -i.bak 's/\ :=\ /:: /g' {} \;

Note that previously, multiple white spaces where allowed before the assignment :=.
To fix up parsing issues, and preserve alignment, I used this (actually I did search replace of the equivalent in my editor, so this is untested):

find . -name '*.som' -exec sed -i.bak 's/\ ::\ /::  /g' {} \;

[coveralls]

Coverage remained the same at 79.897% when pulling b36c759 on setter-sends into c10edd7 on dev.

[ghost]

I haven't looked at all the changes, but it appears that the stream editor has worked correctly. One more step toward compliance!

[smarr]

This solves also the issue in reported in #108. It solves it in the sense that this syntactic ambiguity is gone.

[coveralls]

Coverage remained the same at 79.897% when pulling ec7f929 on setter-sends into 6c3ef7f on dev.