schema: use tuple field names in Lua

When schema.lua was introduced, there was no such thing as space format
and we had to access tuple fields by no. Now we can use human readable
names. Let's do it - this should improve code readability.

A note about box/alter.test.lua: for some reason it clears format of
_space and _index system spaces, which apparently breaks our assumption
about field names. Let's zap those pointless test cases.