doc: rename 'nil' to 'unit' when describing `()`
auto merge of #12210 : zr40/rust/patch-1, r=cmr

According to kimundi on IRC, the current term for `()` is 'unit'. This commit updates tutorial.md to change 'nil' to 'unit' where `()` is described.
Replace `crate` usage with `krate`

This patch replaces all `crate` usage with `krate` before introducing the
new keyword. This ensures that after introducing the keyword, there
won't be any compilation errors.

krate might not be the most expressive substitution for crate but it's a
very close abbreviation for it. `module` was already used in several
places already.
Replace `extern mod` with `extern crate`

This patch adds a new keyword `crate` which is intended to replace mod
in the context of `extern mod` as part of the issue #9880. The patch
doesn't replace all `extern mod` cases since it is necessary to first
push a new snapshot 0.

The implementation could've been less invasive than this. However I
preferred to take this chance to split the `parse_item_foreign_mod`
method and pull the `extern crate` part out of there, hence the new
method `parse_item_foreign_crate`.
Remove obsolete warnings for `extern mod`

This patch gets rid of ObsoleteExternModAttributesInParens and
ObsoleteNamedExternModule since the replacement of `extern mod` with
`extern crate` avoids those cases and raises different errors. Both have
been around for at least a version which makes this a good moment to get
rid of them.