spec: use "core type" rather than "structural type"

This change in terminology prevents potential confusion
that migth be caused by associating "structural type"
with "structural typing"; the two are not connected.

Also, adjusted introductory paragraph of section on
constraint type inference: type inference goes in both
directions, from type parameter to core type and vice
versa. The previous description was not quite accurate.

Change-Id: If4ca300f525eea660f68486302619aa6ad5dbc2c
Reviewed-on: https://go-review.googlesource.com/c/go/+/384238


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>