Initialization of Tables

[gbrebner]

In the BIR specification, rhere is a draft YAML representation of Table initialization (inherited by BIR from AIR), which uses a special-case known identifier: table_initialization. This could be changed to use another, new, BIR object type, with a standard YAML representation. A further possibility is to have a new initialization attribute for Tables (though retaining separate initialization objects too is likely to be useful to decouple initialization as part of the functional code from initialization to provide test data).