Create objects for built-in structures List and String and allow structure.name references

[julianhyde]

In Standard ML, there is a special kind of identifier, called a longid, which may contain dots. For example List.rev is a longid.

Allowing longids would be hard for Morel, because the syntax would conflict with our record.field notation. Instead, we create top-level record values for structures List and String. Thus List.rev is a reference to the rev field of the List.

Later we will remove the underscore-named objects, such as List_rev, from the environment.

[julianhyde]

Fixed in 20d08bd (version 0.3).

Removed underscore-named objects, such as List_rev, from the environment.