`Smalltalk` is global variable, not a class

This is in par with Pharo, where `Smalltalk` is
(single) instance of `SmalltalkImage`, and system things
are accessed via `Smalltalk xxx`.

This also open possibility to create `Smalltalk vm`
to access low-level options and `Smalltalk globals`
as a (JSObjectProxy-ed) dictionary of globals,
thus making Amber API more compatible with other Smalltalks.
Not calling super initialize in class-side initialize
Fixed SetTest >> collectionOfPrintStrings
"Smalltalk globals" returning self, for future compatibility
tests: Smalltalk {current=>globals} at:; Smalltalk {current=>}
Smalltalk vm returning `smalltalk` from `boot.js`
all but IDE: Smalltalk {current=>globals} at:; Smalltalk {current=>}
.st: Smalltalk{ current=>} createPackage:
IDEs: Smalltalk {current=>globals} at:; Smalltalk {current=>}
{Smalltalk,SmalltalkImage} >> current deprecated
boot.js exports globals; HLManager >> defaultEnvironment uses them
Merge pull request #804 from herby/smalltalk-is-global-not-a-class

`Smalltalk` is global variable, not a class