Commits on May 3, 2018

1. Avoid many cmt allocations. …

   `cmt` is a ref-counted wrapper around `cmt_` The use of refcounting
   keeps `cmt` handling simple, but a lot of `cmt` instances are very
   short-lived, and heap-allocating the short-lived ones takes up time.
   
   This patch changes things in the following ways.
   
   - Most of the functions that produced `cmt` instances now produce `cmt_`
     instances. The `Rc::new` calls that occurred within those functions
     now occur at their call sites (but only when necessary, which isn't
     that often).
   
   - Many of the functions that took `cmt` arguments now take `&cmt_`
     arguments. This includes all the methods in the `Delegate` trait.
   
   As a result, the vast majority of the heap allocations are avoided. In
   an extreme case, the number of calls to malloc in tuple-stress drops
   from 9.9M to 7.9M, a drop of 20%. And the compile times for many runs of
   coercions, deep-vector, and tuple-stress drop by 1--2%.

   

   nnethercote committed May 3, 2018
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