Keep locator format¶

Draft. See discussion at #2755

locator       ::= sized-digest hint*

sized-digest  ::= digest size-hint

digest        ::= <32 lowercase hexadecimal digits>

size-hint     ::= "+" [0-9]+

hint          ::= "+" hint-type hint-content

hint-type     ::= [A-Z]+

hint-content  ::= [A-Za-z0-9@_-]*

sign-hint      ::= "+A" <40 lowercase hexadecimal digits> "@" sign-timestamp

sign-timestamp ::= <8 lowercase hexadecimal digits>