design: type parameters: use square brackets
design: type parameters: use square brackets

Also drop the "type" keyword as the start of type parameter lists,
and instead require that constraints always be specified.

Also introduced the predeclared name "any".

Change-Id: If6b005b101cbbaaac73dc2d1c6e40bffaade3a96
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/249840
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/proposal,golang/proposal