Ignoring identity transforms is unjustified

[rptb1]

m7. It seems dangerous to quietly ignore identity transforms, the client might like to know if this means there's a problem in there code. If there's a good reason why ignoring identity transforms meets a requirement, we should refer to it here

Originally posted by @thejayps in https://github.com/Ravenbrook/mps/pull/214#discussion_r1231004242

This needs investigation to discover

• What, if anything, depends on this restriction?
• Is it just a convenient optimisation?
• Does it matter to the client programmer?
• Should it be an error?

[thejayps]

See also #251