runtime/debug: replace (*BuildInfo).Marshal methods with Parse and String

Since a String method cannot return an error, escape fields that may
contain unsanitized values, and unescape them during parsing.

Add a fuzz test to verify that calling the String method on any
BuildInfo returned by Parse produces a string that parses to the same
BuildInfo. (Note that this doesn't ensure that String always produces
a parseable input: we assume that a user constructing a BuildInfo
provides valid paths and versions, so we don't bother to escape those.
It also doesn't ensure that ParseBuildInfo accepts all inputs that
ought to be valid.)

Fixes #51026

Change-Id: Ida18010ce47622cfedb1494060f32bd7705df014
Reviewed-on: https://go-review.googlesource.com/c/go/+/384154


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>