This patch includes packaging changes related to files move.

To pass Go object references to C and back we use hack with casting to
unsafe and then to uintptr.  However, we should not store such references
because Go not guaratnee it will be available by the same address.
Introducing map with integer key helps to avoid dereference stored address.

This closes #253 and #309 issues on GitHub.

This closes #223 issue on GitHub.

Library now used in all language modules.
Old 'nxt_app_*' code removed.

See src/test/nxt_unit_app_test.c for usage sample.

This closes #108 issue on GitHub.

Application should create response for every request.
If not, Unit package will do it.

Passing unsafe.Pointers (void *) from Go to C is complicated by an attempt
to make such pointers less unsafe.

A straightforward optimization is to replace 'unsafe.Pointer' with 'uintptr'
(thanks to Xin Huang for the idea: https://stackoverflow.com/a/44826533 ).

As a result, request registry with mutex is gone.

Go request registration should be removed before C request memory freed.

C request address used as a key in Go map.  Freed memory can be instantly
reused for other request and older request registration should removed at this
point to avoid collisions.