Fix rust-align-to-expr-after-brace, closes #11239.

forward-to-word is undefined, and so Emacs would throw errors in
rust-align-to-expr-after-brace. This change yields the expected
behavior discussed in the issue.
auto merge of #11244 : johnwalker/rust/issue-11239, r=brson

forward-to-word is undefined, and so Emacs would throw errors in
rust-align-to-expr-after-brace. This change yields the expected
behavior discussed in issue #11239 by handling the case when
whitespace follows a left bracket.