Array backend additional TODO list

[jangko]

• fix compiletime test_uint_muldiv
• fix compiletime test_uint_modular_arithmetic
• fix compiletime test_int_muldiv
• test against nimbus-eth1
• test against nimbus-eth2
• stuint truncate implementation and test
• conversion from stuint to stuint
• conversion from stuint to stint
• conversion from stint to stint
• conversion from stint to stuint
• conversion test suite
• signed int modular arithmetic and test, what important is the stuint backend
• comments scrutiny, remove/fix misleading comments, and add comments to unclear operations
• stuint endians2 more comprehensive tests
• stint endians2 implementation and tests
• signed int arith overflow and test
• add native signed integer operand to StInt addsub operations
• add native signed integer operand to StInt muldiv operations