Procedure & (lambda) function as extra instructions & computations

[nsensfel]

Add two first level instructions:

(define_computation <return_type> <name> (<var list>) <computation>) which defines a lambda function that is registered as an extra computation.

(define_instruction <name> (<var list>) <instruction list>) which defines a sequence that is registered as an extra instruction.

Add a warning if those would collide with an existing computation/instruction, but then replace the existing one.

[nsensfel]

I think it might be better to actually have:

• (register_computation [LAMBDA] {name: String})
• (register_instruction [SEQUENCE] {name: String})