box.space compatible API

[no1seman]

As DDL module is a high-level interface for compatibility of different versions of Tarantool it must have API maximally compatible with the structure of box.space*. Now ddl.get_schema() have the following problems (the list may not be complete):

1. ddl.get_schema().spaces[] whereas box.space[]
2. ddl.get_schema().spaces[].indexes whereas box.space[].index
3. ddl.get_schema().spaces[].indexes doesn't contain an id field
4. ddl.get_schema().spaces[] doesn't contain an ck_constraint description