In case the warning -Werror=conversion is active with GCC, the warnings
about "conversion from A to B may change value" lead to a compilation
error. This explicitly convert the values to address these warnings.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

It used to be part of the experimental benchmarking support, but
since that was replaced with proper benchmarking support with its
own timer facilities, it is now a dead code and useless.

This should also be done for test files, but that has lower priority.

This is both a really big and a really small commit. It is small in
that it only contains renaming, moving and modification of include
directives caused by this.

It is really big in the obvious way of touching something like 200
files.

The new rules for naming files is simple: headers use the `.hpp`
extension. The rules for physical file layout is still kinda in
progress, but the basics are also simple:
 * Significant parts of functionality get their own subfolder
   * Benchmarking is in `catch2/benchmark`
   * Matchers are in `catch2/matchers`
   * Generators are in `catch2/generators`
   * Reporters are in `catch2/reporters`
   * Baseline testing facilities are in `catch2/`
 * Various top level folders also contain `internal` subfolder,
   with files that users probably do not want to include directly,
   at least not until they have to write something like their own
   reporter.
    * The exact files in these subfolders is likely to change later
      on

Note that while some includes were cleaned up in this commit, it
is only the low hanging fruit and further cleanup using automatic
tooling will happen later.

Also note that various include guards, copyright notices and file
headers will also be standardized later, rather than in this commit.