After rustwasm#1594 constructors of Rust exported structs started using class
wrapping when generating JS shims. Wrapping erases prototype information
from the object instance in JS and as a result it is not possible to
override methods (via inheritance) of the generated class.

Additionally, some checks to ensure constructors always return an
instance of `Self` were lost.

This PR is rebased from rustwasm#3561, it passes the constructor information
from the `Builder` into the instruction translation function which
is then used to modify the generated bindings.

The `return` statement is also removed instead the value is directly
attached to the instance.

Signed-off-by: Oliver T <[email protected]>

Co-authored-by: Liam Murphy <[email protected]>

…orts

Signed-off-by: Oliver T <[email protected]>

Signed-off-by: Oliver T <[email protected]>

Signed-off-by: Oliver T <[email protected]>