Try to get rid of evaluated imports in generated import libraries

[sjamaan]

As we found out in #1457 and #1172, merge-se is quadratic in complexity. This means that every import we do is potentially really slow.

Now that we only emit eval calls when generating import libraries of modules with syntax exports, it might be worthwhile to see if we can get rid of them altogether. As ​proposed earlier, one way to do this would be to attach the syntax environment as an alist to each macro in the import library, instead of deriving it via imports (which will actually provide a much larger environment than necessary).

[sjamaan]

Getting ready for 5.1, moving tickets which won't make it in to 5.2.