type check lambdas when last op (body) is set

[leissa]

• finally type checking for mutable Lams
• fixed bug in alpha-equiv - related to #174?
• fixed bug in lower_matrix_mediumlevel.cpp (wrong filter type)
• fixed type inference for things like umax(0, ?) == 1

[leissa]

@NeuralCoder3 can you double-check my changes in lower_matrix_mediumlevel.cpp?

[NeuralCoder3]

The build of bfdcd46 seems to fail. Maybe the filter can not be true? It seems to be an unrelated problem with internet connections.