Rename field type names, deprecate 'num'

* Rename fields types in data dictionary and test suite:
 - 'num' => 'unsigned'
 - 'str' => 'string'
 - 'int' => 'integer'
* Force coversion to lowercase
* Add aliases to space:create_index():
 - 'str' => 'string'
 - 'num' => 'unsigned' -- logs "field type 'num' is deprecated" warning
 - 'uint' => 'unsigned'
 - 'int' => 'integer'
* Add prefix to `enum field_type` members
* Add upgrade_to_1_7_2() script

Fixes #942
Fixes #1534