nll part 5
Next round of changes from the nll-master branch.
Extensions:
- we now propagate ty-region-outlives constraints out of closures and into their creator when necessary
- we fix a few ICEs that can occur by doing liveness analysis (and the resulting normalization) during type-checking
- we handle the implicit region bound that assumes that each type `T` outlives the fn body
- we handle normalization of inputs/outputs in fn signatures
Not included in this PR (will come next):
- handling `impl Trait`
- tracking causal information
- extended errors
r? @arielb1
