Restore OOM test (TreeIndex moved to another arena).
Update digest.result (spaces)
Update net.box.result (line numbers).

* Removed b64decode from box_net_box.lua
* Added base64_encode() and base64_decode() methods in digest
* Added base64 tests

All bultin modules can be loaded with require('modulename')
instead of require('box.modulename')

Other changes:

 * _G.box is not initialized from tarantool_lua_init() anymore
 * box.info, box.stat, box.snapshot moved to src/box/lua
 * box.coredump() => coredump()
 * box.xxx = require('box.xxx') removed from tests

Test wasn't correctly merged from 1.5