[MERGE] SourceLayout: esi, take 1

SourceLayout: esi/, take 1

Moved src/ESI* files into src/esi/.
Renamed ESI source files from ESIFoo.{cc,cci,h} to Foo.{cc,cci,h}.

Replaced implicit constructor-based Parser registration with an explicit
call
to Esi::Init() which knows of all ESI parsers. Added Esi::Clean counterpart.

Made libTrie build conditional on ESI support being enabled.

No true source code changes except for parser initialization.
No functionality changes were intended.
No runtime tests with ESI performed.

TODO: I did not move the tests/ESIExpressions test into src/esi because
we need to fix the entire testing layout before we should start
moving individual tests: They currently depend on individual source
and object files in src/ and in tests/ and that kind of dependency
should not be multiplied but removed.

TODO: Move ESI types into the Esi namespace.

Please review and, if you know how to use ESI, test.

Thank you,

Alex.
bb:approve