add fuzz test for FromString

[acln0]

Fuzz FromString using github.com/dvyukov/go-fuzz. FromString calls
(*UUID).UnmarshalText, so this test touches all of the places where
package uuid might parse interesting user input.

Since the initial encoding of the UUID is lost when parsed, we cannot
do any better than a simple valid/invalid test.

Updates #2.

[niaow]

note: CI failures will be fixed by #4

[theckman]

@acln0 To confirm, we're intending on attacking the TODO at a later time?

[acln0]

@theckman Yes. I wanted to just get this in for now. Once we figure out the answer to the question of where to store the testing corpus (if at all) in #2, I'd like to discuss how to seed the corpus with inputs we're already using for testing.