Support the interface construct

[stadelmanma]

Interface blocks are another standard language feature.

References:

• http://www.lahey.com/docs/lfpro78help/hh_start.htm#F95LSProcedureInterfaces.htm
• http://www.lahey.com/docs/lfpro78help/hh_start.htm#F95ARMODULE_PROCEDUREStmt.htm