subst-rv, subst-st, subst-bk. partial work on fresh lifetimes for block
activation.
Lifetime substitution is now noncapturing.
Currently gives up when capturing could occur instead of auto
α-converting. Breaks several tests that assumed lifetime names
are not replaced when a block is activated.
subst-ℓ should change nothing when the lifetime has no substitution.
Capture avoiding lifetime substitution for blocks.
Consolidate capture-avoiding substitution for blocks.
Deterministic fresh lifetime generation for block activation.
Deterministic fresh lifetime generation for function calls.
Substitute actual lifetimes for formal lifetimes in function calls.
Deterministic fresh lifetime generation for match.
Merge pull request #9 from anasazi/fix-block-fresh-lifetimes

Deterministic Fresh Lifetime Generation & Capture Avoiding Substitution