lift and deduplicate constants

(descoped from #773)

Given a potentially hot function, we should not allocate data in the function which is known to be constant.

Define a "deep constant" to be a closLang expression consisting entirely of Const (integers), String, WordFromInt, and Cons (i.e. all children of a deep constant must be deep constants). Define a "nontrivial maximal deep constant" to be a deep constant which is not contained in a larger deep constant and which requires allocation (is not a Cons with 0 children or a Const in fixnum range.

The (symbols PR) compiler has:
* 57 nontrivial maximal Const, 5 unique
* 3336 nontrivial maximal Cons, 2058 unique
* 866 nontrivial maximal String, 589 unique
* 2254 nontrivial maximal WordFromInt, 121 unique

Total: 2773 unique liftable constants used 6513 times. Of the constants used more than once, there are 624 unique constants; the largest duplicated constant has 39 subobjects (it appears to be related to the initial type environment), and only 697/3740 duplicated constants involve more than one heap object.

Doing only lifting would be a performance improvement but a code size regression since each lifted constant needs to be allocated a global, stored, and loaded (20+ instructions currently, 4 after #797). With deduplication as well this will be marginal currently and a noticeable win with #797.

With the current setup of the semantics this has to be done in closLang as later stages represent strings non-anonymously.