Add coerced type arguments.

[python-processing-unit]

Add coerced type arguments.

During the declaration of a FUNC (including LAMBDA), if an argument's type declaration is prefixed with ~ it MUST be a coerced type argument.

For normal (non-coerced) arguments, if the type of the supplied argument and the expected type differ, it MUST error. For coerced type arguments, however, if the supplied argument can be converted to the expected type, it MUST be converted, and it MUST NOT error. If the supplied argument cannot be converted to the expected type, it MUST error.

Update both the implementation and the specification.

[python-processing-unit]

Also, ban ~ from symbols.