PLCgen: use smallest integer representation possible for ranged integer and enum-typed variables/constants
Use smallest integer representation possible for ranged integer and enum-typed variables/constants ([BYTE](https://support.industry.siemens.com/cs/mdm/89515142?c=56595553163&lc=en-CR)/INT instead of DINT, etc). See also representation [list of supported types in S7](https://support.industry.siemens.com/cs/mdm/89515142?c=58906052747&lc=en-CR).
See also discussions in #912.
Addresses #912, #679
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