Particular formatting improvement in the spec

Fixes the titularity of the "Overloading resolution" header
under 6.26.1 "Value conversions" to match "Type instantiation."

Hopefully this makes the link to the actual 6.26.3 overloading
resolution section work without further renaming. (Edit: changed
the minor heading to say "Static overload resolution", which is
sufficiently redundant.)

Possibly that is the oftenest-cited section of the entire document.

At least on Stack Overflow, if not in Proceedings of the ACM.
Merge pull request #3990 from som-snytt/issue/spec-overload-title

Particular formatting improvement in the spec
Make compiler.properties fall back to prefixed

Previously, if we wanted to override the shell.prompt property, we had
to modify compiler.properties in the jar. This change lets us do the
following, instead:

    scala -Dscala.shell.prompt="$(echo -e "\npuffnfresh> ")"

All properties previously loaded from compiler.properties now fall back
to "scala." in the system properties when not found.
HasSet to HashSet typo fix
Merge pull request #3997 from krystiannowak/patch-1

HasSet to HashSet typo fix
Merge pull request #3994 from puffnfresh/feature/system-compiler-properties

Make compiler.properties fall back to prefixed
Rename ClassPath.findSourceFile to ClassPath.findClassFile

If you look at the implementation of that method and its usage
its clear that it should have been named `findClassFile` from the
beginning because that's what it does: find a class file and
not a source file.
Get rid of Platform.doLoad method.

Since .NET backend got removed this method is a no-op.