Improve handling of references to Object coming from Java code

[smarter]

In Java unlike Scala, Object is the top type, this leads to various
usability problems when attempting to call or override a Java method
from Scala. So far, we relied mainly on one mechanism to improve the
situation: in the ClassfileParser, some references to Object in
signatures were replaced by Any (cf objToAny). But this had several
shortcomings:

• To compensate for this substitution,
  TypeComparer#matchingMethodParams had to special case Any in Java
  methods and treat it like Object
• There were various situation were this substitution was not applied,
  notably when using varargs (Object... args) or when jointly
  compiling .java sources since this is handled by JavaParser and not
  ClassfileParser.

This commit replaces all of this by a more systematic solution: all
references to Object in Java definitions (both in classfiles and .java
source files) are replaced by a special type FromJavaObject which is a
type alias of Object with one extra subtyping rule:

 tp <:< FromJavaObject

is equivalent to:

 tp <:< Any

See the documentation of FromJavaObjectSymbol for details on
why this makes sense.

This solution is very much inspired by
scala/scala#7966 (which was refined in
scala/scala#8049 and
scala/scala#8638) with two main differences:

• We use a type with its own symbol and name distinct from
  java.lang.Object, because this type can show up in inferred types
  and therefore needs to be preserved when pickling so that unpickled
  trees pass -Ycheck.
• Unlike in Scala 2, we cannot get rid of JavaMethodType because when
  calling Type#signature we need to know whether we're in a Java
  method or not, because signatures are based on erased types and erasure
  differs between Scala and Java (we cannot ignore this and always
  base signatures on the Scala erasure, because signatures are used
  to distinguish between overloads and overrides so they must agree
  with the actual JVM bytecode signature).

Fixes #7467, #7963, #8588, #8977.

[odersky]

LGTM, except for the missing explanation of the puzzler in translateJavaArrayElementType.

[smarter]

Addressed comments by updating the documentation of FromJaveObjectSymbol and translateJavaArrayElementType, as well as adding an extra commit which introduces Context#isJava and uses it where possible.

[retronym]

Thanks for aligning this, @smarter.