Fix crash when handling parameters with invalid types

Fixes a compiler assert when checking a parameter for autorecovery that has an invalid type.

Fix some infinite loops during typechecking

Some recursive constraints could cause the typechecker to go into an infinite loop while trying to check subtyping. These infinite loops have now been fixed. An example program that demonstrates the issue is below:

primitive Foo[T: (Unsigned & UnsignedInteger[T])]
  fun boom() =>
    iftype T <: U8 then
      None
    end

actor Main
  new create(env: Env) =>
    Foo[U8].boom()

The use of T: UnsignedInteger[T] is valid and expected, but wasn't being handled correctly in this situation by the typechecker in this context.

Fix a major bug which allowed mutating fields of a val object

A bug was introduced in 0.51.2 which caused some checks to allow mutating fields of a val object due to changes to some operations on capabilities. This bug is fixed: immutability is now checked separately from other properties.

If you are using Pony 0.51.2 to 0.52.3, you should upgrade to the 0.52.4 release that contains this fix as soon as possible.