Update HTML files of examples for 2.11.0.
Refactor hierarchy of virtual files to VirtualTextFiles.

The current virtual files are implicitly text files, but
eventually we will need to also represent virtual binary
files. This commit refactors the hierarchy to separate
VirtualTextFile (text) from VirtualFile (agnostic). This
refactoring will allow the later introduction of binary
files as VirtualBinaryFiles.
Merge pull request #507 from sjrd/html-files-2.11

Update HTML files of examples for 2.11.0.
Merge pull request #509 from sjrd/refactor-virtual-files

Refactor hierarchy of virtual files to VirtualTextFiles.
Emit serialized IR files instead of .js + .js.map + .sjsinfo.

The ClasspathBuilder automatically expands .sjsir files into their
desugared VirtualScalaJSClassfile version for now. It is left for
a future commit to get rid of VirtualScalaJSClassfile entirely and
have the packager and optimizer work directly on the IR file.
Remove VirtualScalaJSClassfile and use VirtualScalaJSIRFile instead.

Now the various tasks working with a ScalaJSClasspath extract the
information they need from the IR directly, and work with that.
Cache the desugared pieces of classes in the fast optimizer.

This way, info and trees of .sjsir files that have not changed
since the last invocation of fastOptJS need not be reloaded from
disk. Instead, they are kept in-memory. For the trees, the
already desugared is stored (and only pieces actually used).
Merge pull request #511 from sjrd/push-ir-until-linking

Push the IR until linking
Minor cleanup (unneeded parens and braces)
Get rid of genCallHelper in GenJSCode
Improve state management in GenJSCode