reorganize tests

[stepancheg]

Move all tests into subfolders.

• lang -- language and compiler tests, including primitives
• lib-clay -- stdlib tests
• lib-clay/core -- tests of core.*
• example -- tests that are example of what is Clay capable of
• example/shootout -- moved from test/shootout

This is necessary to quickly find out which parts of the project are covered with tests.

The downside of this separation is that sometimes tests of similar entities lies in different folders, like lang/floats and lib-clay/floats.

Patch contains nothing but file renames. No test files are modified or removed.

[jckarter]

Much nicer. Thanks!