Draft strict mode

[guillep]

Draft an optional strict code loading mode that does not accept:

• undeclared variables
• syntax errors

Instead of just loading the code with errors, this option makes loading fail with an exception.

This PR implements this through:

• extending monticello loaders with a strict flag
• extending metacello with
  • the ability to specify the strict loading of a baseline/package
  • the ability to change the default strictness for a whole loading in the Metacello new ... strict: true expression

[Ducasse]

I would like to have a strict mode where we cannot redefine self and others too dynamic features.