Rename `col!` to `column!`.

This macro is very rarely used, so there is no need (and it is better)
for it to avoid the abbreviation.

Closes rust-lang/rfcs#467.

[breaking-change]
auto merge of #19071 : huonw/rust/col2column, r=nikomatsakis

This macro is very rarely used, so there is no need (and it is better)
for it to avoid the abbreviation.

Closes rust-lang/rfcs#467.