Escape ';' in N and ADR structured component values

[gaoflow]

The bug

A literal ; inside a structured-value component (N, ADR) is silently corrupted on a round trip through the library's own API:

c := make(vcard.Card)
c.SetValue(vcard.FieldVersion, "4.0")
c.SetName(&vcard.Name{FamilyName: "a;b", GivenName: "c"})
// encode -> "N:a;b;c;;;"  (the ';' in "a;b" is emitted as a raw separator)
// decode -> Name{FamilyName: "a", GivenName: "b", ...}

FamilyName comes back as "a" and "b" leaks into GivenName. The encoder writes the literal ; unescaped, and on decode it is read (correctly, per spec) as a component separator.

Root cause

RFC 6350 section 3.4 defines a component escape set of \\, \,, \; and \n; a ; separates components only when unescaped. Name.field() / Address.field() join components with a raw ; and newName / newAddress split on a raw ;, while the value escaper handled \\, \n and \, but never the sibling \;. So a literal ; in a component has no escaped representation and round-trips as a separator. \ and , are handled and round-trip fine — ; is the missing case.

Fix

Escape and unescape structured values per component (backslash, newline, comma and semicolon) and join/split on the unescaped ;. A literal ; now serialises as \;:

N:a\;b;c;;;

Because a component is escaped as a unit, the general value escaper must not run a second time over N/ADR, so the encoder and decoder leave those to the structured path. One consequence worth calling out: the raw Field.Value of an N/ADR field is now the escaped structured value (e.g. a\;b;c;;;); the components are obtained through Name() / Address() as before. Parameter values are deliberately untouched — they are quoted, not backslash-escaped, and are not structured values.

Tests

structured_test.go covers round-trips of ;, ,, \, \n, a backslash at a component end and a literal \;, for both N and ADR, asserts the encoded form (N:a\;b;c;;;), and asserts a ; in a parameter is not structurally escaped. Full suite passes; gofmt and go vet clean.

Related but not addressed here: ORG and CLIENTPIDMAP are also ;-separated but have no typed accessors, and CATEGORIES has the same separator-vs-literal question for ,; happy to follow up if you'd like those in scope.