Fix two minor issues in the tutorial.

Remove an extraneous 'then', and replace 'second' with 'first'.
Add option::get_zero

r=brson

Closes #3797
core: fix windows breakage, r=burningtree.
Make either::{partition, flip, to_result} take their arguments by-value

Addresses an XXX

r=pcwalton
Rename option::get_default => get_or_default, get_zero => get_or_zero
Merge remote-tracking branch 'superoptimizer/master' into HEAD

Conflicts:
	doc/tutorial.md
Add Mark Lacey to AUTHORS
librustc: Stop generating first-class aggregates in visit glue, since they kick us off fast isel. Closes #4352. rs=minor-perf-improvement