shorten import alias code
fix import name check for import aliases
fix type matching

MatrixType must have precedence over MathematicalType, because
it may start with a MathematicalType.
add metamodel for explicit cast expressions
add syntax for explicit cast expressions
adjust TypeCheckExpression syntax to be consistent with casts
type cast expressions
add small cast / type-check example
validate that a cast is possible
implement interpretation of explicit casts
Merge branch 'casts'

Closes #20.