Support printing spans, especially in tests.

This allows EXPECT_EQ(span, span2) to print more useful output on a
failure. The span<char> case is specialized to print the contents as a
string instead of each char in an array.

Remove an operator<< implementation for span from the
shape_result_bloberizer in blink, which helps demonstrate the need for
this support.

Bug: 40285824
Change-Id: I679aa5ae1c26bfa250ee8f899a6d6e11770d767f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5604849
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Owners-Override: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1312855}