add Tuple class to core
specify model classes in XCore
add XSemantics logic for tuples
add syntax for tuples, tuple types and parallel assignment to tuple components
add some syntax examples for tuples
tuple assignment: allow some components to be left empty
add "tuple var declarations"

This is a way of declaring multiple variables and assigning
them to the entries of a tuple. The types can be inferred from
the tuple's type or be specified on the individual variables.
variables declared in tuple declarations are always initialized
fix scoping for TupleVarDeclarations
adjust var initialization check for tuples

Recognize tuple assignments when checking if a var is initialized.
Also, recognize any assignments inside a tuple expression.
interpret tuple expression and tuple assignment
implement interpretation of tuple var declaration
implement implicit coercion and type checks for tuples
change Tuple.toString() output
Merge branch 'tuples'

Closes #16.