The alternative, flat representation of classpath elements

This is another PR created for the flat classpath representation. This time the target is changed to 2.11.x branch.

It's finished work started and abandoned by @gkossakowski
Related discussion: https://groups.google.com/forum/#!msg/scala-internals/xgb5YbKvQcw/7bcbsvKkzQkJ

The main goal of Grzegorz was to improve the efficiency of classpath. At the end it turned out that the efficiency improvement is not significant so Grzegorz stopped working on that. 

On the other hand it was worth to continue these works due to several reasons:
- it's possible to reduce memory consumption significantly
- each possible efficiency improvement is important
- the old classpath is maybe too complex because it was created when e.g. .NET was supported - it can be simpler

New (flat) classpath has dedicated classes for various file types characteristic for Scala with JVM backend - what allows us to create more efficient, more low-level operations.
To reduce memory consumption there's added caching for jar/zip files. So that we can reuse existing instances, when we create entries for the same files once again.

One of the most important things took into account during creating this implementation was adding it as an optional feature, which can be marked as experimental and turned on using flag. At the beginning users will use an old classpath implementation by default and they will be able to use the new implementation when:
- they will want to test it (feedback)
- they will want to gain benefits which it provides

I took into account also planned better support for JSR-223. So it supports for instance ManifestResources introduced here: https://github.com/scala/scala/pull/2238
There was no proper description what's going on with these changes and how to test them but @rjolly sent me some useful papers.
This particular type of classpath can be tested using e.g. command:
jrunscript -classpath scala-compiler.jar;scala-reflect.jar;scala-library.jar -l scala
where scala-library.jar was modified using https://github.com/rjolly/jarlister

I tested flat classpath in several ways:
- First of all obviously all tests (there are added certain new ones especially for this implementation) pass and bootstrapping also works.

- I built Scala IDE using my version of Scala and tested it with a very large codebase with about 100 projects. I turned off the closing of unused presentation compilers which I added lately. And after that I did certain custom operation which does a deep refresh of all projects (close projects, regenerate the workspace, open projects).

The flat classpath with turned on caching allowed me to reduce allocated memory in old gen from about 6GB (for current classpath) to about 750MB. So in the case of IDE and presentation compilers the gain is really significant.

- The efficiency of both implementations in real use cases seems to be similar but probably it could be tested better. In small benchmarks the flat one was winning.

- On the other hand there are situations when the gain is quite significant. When running a whole Scala's test suite:

Tests using a computer with SSD:

| Time type:|      recursive classpath      | flat classpath|
| ------------- |:-------------:| -----:|
| avg of 4 repetitions     | 29 minutes 34 seconds | 27 minutes 15 seconds |
| min     | 29 minutes 28 seconds     |   27 minutes 6 seconds |
| max | 29 minutes 41 seconds      |    27 minutes 32 seconds |

Tests using other computer with the scala project directory moved to some external HDD connected via USB cable:

| Time type:|      recursive classpath      | flat classpath|
| ------------- |:-------------:| -----:|
| avg of 4 repetitions     | 56 minutes 43 seconds | 52 minutes 38 seconds |
| min     | 55 minutes 40 seconds     |   52 minutes 8 seconds |
| max | 58 minutes 26 seconds      |    53 minutes 0 seconds |

Note: I changed the classPath method in Global to the dynamic dispatch. And now there's returned a base class of the old and the new classpath with common interface based on a part of API of the old classpath. I tested it with sbt and there's no problem with compiling projects etc. Previously Grzegorz created certain hack for sbt's compiler interface where current classpath was calling the flat one, when it was needed. I changed this ugly solution to this mentioned dynamic dispatch and removed a hack. We can return to something like this, if it'd turn out that it's necessary.

Thanks to Grzegorz for hints and his initial implementation.

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