Loop counter variables

[dloscutoff]

A loop statement (currently any of F, L, LR, T, and W) should automatically set a local variable to its 0-indexed current iteration count. After the loop completes, the variable should equal the number of times the loop ran (possibly zero). The outermost loop(s) in a local scope should use e as their counter variable; the next level of nested loop(s) should use d, then c, then b, then a.

(See further discussion at #70.)