implicit receiver syntax & supporting changes

[nikodemus]

• Unary methods with implicit receiver indistinguishable from variable accesses. (Variable and slot lookups could be considered unary sends to the lexical environment, but this semantic distinction need not be made now.)
• Keyword methods allow function-like methods to be defined outside classes at module level.
• Implicit receiver always resolved lexically: local environment, current class, current module.

Supporting changes:

• assignment uses := instead of =. Needed to disambiguate one-element records from blocks with an assignment.
• records use = instead of keyword syntax: { foo = 42, bar = 123 }. Need to disambiguate one-element records from blocks with an implicit receiver keyword send.
• balanced delimiters around method bodies: needed to disambiguate first expression like foo: bar from additional method keywords.
• defining methods at toplevel of module: method sin: angle ...
• importing module and direct methods: import #sin: from lib.math