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.)