Extension methods & conversions for Java host types

[JaroslavTulach]

Based on the #8346 (comment) idea.

• Runtime checks of signatures with polyglot java classes #8016

Since #7858 was implemented, we can use polyglot java import types (e.g. host Java classes) in type ascriptions. The host Java classes are recognized in Meta.type_of and we can even use them in case of statements:

polyglot java import java.math.BigDecimal

c x = case x of
  _ : BigDecimal -> "big"
  _ -> "small"

However there are still few areas where the support of host Java classes isn't on par with Enso types - one cannot add extension methods to them and one cannot define conversion methods. Both of these features are technically possible and they would smooth interoperability between Enso and Java when designing the Enso libraries.

[JaroslavTulach]

Now, when the attempts to compile some of the libraries by native image are discussed at

• Launching Enso programs instantly #10121

it makes little sense to invest in making Java types first class citizens in Enso. As it is not going to be possible to exchange an object from NI with object from JVM - even if they have the same java.lang.Class they will be different.