Add support for generated fields

Should allow specifying when to generate:

- OnCreate
- OnLoad
- OnEdit

Example generators:

- DateTime (with format specifier)
- MD5 (with multi field specifier)
- FNV1 (with multi field specifier)
- UUID
- Tally
- Arithmetic (operand A, operator, operand B)
- Comparison (operand A, operator, operand B)
- Expression (evaluated C# lambda)