Typo s/recusive/recursive/
Add and lex LIFETIME tokens

cc #4846
auto merge of #4845 : osaut/rust/master, r=graydon

There was a simple typo on recursive in doc/tutorial-macros.md: s/recusive/recursive.
auto merge of #4844 : jld/rust/const-index-fix, r=graydon

Fixes #3169 and uses that to fix const array indexing, which I broke (in a way that doesn't break tests but will greatly confuse users) while trying to fix const enum vectors.
auto merge of #4847 : nikomatsakis/rust/region-syntax, r=graydon

cc #4846

r? @pcwalton
Omit discriminant from nullary univariant enums.

If an enum is isomorphic to unit, there's no need to use any bits to
represent it.  The only obvious reason this wasn't the case was because
the enum could be C-like and have a user-specified discriminant -- but
that value is constant, so it doesn't need to be stored.

This change means that all newtype-like enums have the same size (and
layout) as their underlying type, which might be a useful property to
have, at least in terms of making programs' low-level behavior less
surprising.