Type inference: follow-up tasks

[sharkdp]

see #443 for background information

• Update documentation #523
• Fix bug with type parameter name clashes: fn f<A: Dim>(x: A) -> A * Velocity = x * c does not work, giving A a different name works fine.
• Improve startup time (regression) #525
• Pretty-printing: do not show forall multiple times #524
• Better type name generation policy: (1) handle name clashes with existing types (e.g. struct names), (2) try not to show things like T423 to a customer, but rather reset the counter after type-checking a single statement — if possible.
• Something like fn foo<D1, D2>(x: D1*D2) = 1 should yield a "missing Dim bound" constraint, because the type parameters are used type annotation dimension expression
• Re-enable generics_unused_type_parameter
• Equality on function types should be forbidden. Maybe we need to introduce an Eq type class.
• try_trivial_resolution can easily analyze the constraint structure further to discard constraints like List<T0> ~ Bool, which will yield better error messages in return
• Better error message for sth. like true(2)
• Check if we can reduce the number of constraints: (1) by cancelling out equal constraints and (2) by not adding as many constraints. for example, in x + y, we need type(x) ~ type(y) and IsDim(type(x)), but we could potentially save the IsDim(type(y)). On the other hand, that would result in worse error messages.
• For all (generic) functions that we have, check that we can (1) remove all type annotations and (2) get the same type or a more general type.
• Add tests for type inference with partial annotations
• Better error message for fn f<A: Dim>(x: A) -> A = 2x; f(true)
• Improve error messages for callables, see function_types_in_argument_position test
• Print parameter names in info <function> #491
• Bring back (and improve) "readable types" #490
• update https://numbat.dev/articles/intro.html
• Re-read http://typesatwork.imm.dtu.dk/material/TaW_Paper_TypesAtWork_Kennedy.pdf
• Implement mean, stddev, etc in Numbat
• Give quadratic_equation a List<B/A> return type. #449
• When quantifying a function type with annotations, use the user-specified identifiers
• Bring numerical diff functionality to the prelude (Numerical derivatives, integrals #145)
• Use assert_eq instead of assert in Numbat code tests