Handle delay, function, and/or command types in read

[byorgey]

As implemented in #2224 , the read command can only handle first-order values with no captured environment, which excludes delay types, function types, and commands. In some cases, however, handling these would be straightforward, for example format move == "move" and read "move" : Cmd Unit should be easy to implement. In other cases it is a bit tricker, and might even require fixing some bugs in format. For example, currently we pretty-print function closures using let bindings:

> def x = 3 end
> format (\y. y + x)
it0 : Text = "let x = 3 in \y. y + x"

In order to read such values we would need to specially handle let bindings to turn them into an environment (or possibly simply run the resulting Term through the interpreter until it becomes a Value --- but we would have to be careful to only do this for terms that we can statically tell will really turn into values in finite steps, since otherwise read would essentially be eval and could cause the game to hang).

In other cases, however, it looks like format doesn't actually do the right thing. For example, delayed values can also close over the environment, but this does not currently get formatted with let:

> def z = 2 end
> format {z + 3}
it1 : Text = "{z + 3}"