Single-File: MemoryMap the bundle file (dotnet/core-setup#6576)

Single-File: MemoryMap the bundle file (dotnet/core-setup#6576)

* Single-File: MemoryMap the bundle file

This changes makes a few changes in preparation for
loading IL files from the single-file bundle.

**Memory Mapping**

At startup, the entire bundle-file is memory mapped.
The individual embedded files are then extracted out to actual files on disk on startup.
However, this change will facilitate later checkins to:
* Loading files directly from bundle (ex: IL files using PELoader, config files using memory-streams, etc ).
* Extracting out files to disk lazily
Further details are available in [this document](https://github.com/dotnet/designs/blob/master/accepted/single-file/design.md#the-host)

**Refactoring**

* Static bundle_runner: The bundle_runner is made a static class that holds a global pointer
  to the memory-mapped application bundle, and meta-data about the contents.
  This facilitates the implementation of call-backs for accessing content directly from the bundle,
  as explained in [this document](https://github.com/dotnet/designs/blob/master/accepted/single-file/direct.md)

* A few new classes are created by factoring out functionality from bundle_runner
   * Extractor: Functionality for extracting contents of a bundle to files on disk.
   * Reader: This is a helper class that implements sequentional reading on memory-mapped bundle file.
   * dir-Util: Extraction directory handling utilities that assist in processing the bundle.

* Naming: bundle::bundle_runner is renamed as bundle::runner to remove the redundancy.
  Similarly, the newly refactored classes are named bundle::extractor and bundle::reader.

Commit migrated from https://github.com/dotnet/core-setup/commit/9bb119b2a4b82d54e73669928a3b831cc07a1291