Add type checker

[foresterre]

• The type checker must ensure scripts and cli options are type compatible

  • for example: an inform action doesn't return an image buffer
  • for example: resize x: Into<SizeUnit> y: Into<SizeUnits> takes for both x and y a certain size unit. This may be a pixel, but also a percentage as propose in Add unit parsing for operation arguments #600. Our type checker should be able to verify the type can be taken by the resize fn

• We should be able to run the type checker on "compile time", that is before a user ever supplies an actual image