Rename primitives for clojure compatibility

[jar600]

Most primitives (see builtin.clj) got their names from the Python version of metaprob. I was hesitant to change them lest we lose python compatibility. But now, having dropped that requirement, we could rename them.

For example, the arithemetic primitives are add, sub, mul, div, le, gte and so on. They could be + - * / < >= and so on.

A few other clojure inconsistencies are: length/count, tuple/vector.

More generally, there has been no overall review of the language design now that we are dropping the Python compatibility requirement.

[vkmvkmvkmvkm]

Please spend time doing this soon --- before POPL.

[jar600]

How far to go with compatibility? E.g. what about eliminating define in favor of a combination of let and letfn ?

[jar600]

This is mostly done. New names are available and instituted through much of the code. Old names preserved temporarily. Left infer.clj alone pending merge of #25.

[jar398]

I'm going to close this. length is not really the same as count because count of a nonempty list has to always return 1 (only 1 subtrace key) while length has to return its length. Changing tuple to vector is a big pervasive change and should be done separately. block vs. do is discussed in #59. We talked about doing a comprehensive review and this will come again then.