Restructure handling of classes and methods in mapper/cache

[loewenheim]

Currently, when parsing proguard mapping files, we don't "abstract" classes and methods; we simply take lines of the proguard file as they come. This has some negative consequences. Consider this mapping example:

some.package.Class -> foo.a:
# {"id":"sourceFile", "fileName":"Foobar.kt"}
    4:4:void example.Foo.lambda$main$0():225 -> a
    4:4:void some.package.OtherClass.run(example.Foo):2 -> a
    4:4:void run():3 -> a
    5:5:void run():4 -> a
    # {"id":"com.android.tools.r8.synthesized"}

some.package.OtherClass -> foo.b:
# {"id":"sourceFile", "fileName":"Other.kt"}
    1:10:void run(example.Foo):1:10 -> a

1. The file name for OtherClass should be Other.kt. When remapping a stack frame for e.g. foo.a.a:4, we just take the file name of the class the method is called in (Foobar.kt), which is incorrect.
2. With respect to Support synthesized class annotations #48: The 5:5 mapping of Class.run is marked as synthesized. This should affect all mappings for the method, but it currently doesn't because we don't keep track of methods.