Support a few more type mappings in the compiled model

[roji]

Part of #2949

[roji]

@AndriySvyryd here's some Npgsql-specific customizations to type mapping code generation in the compiled model, this causes an additional Clone() to be generated with the required stuff:

foo.TypeMapping = NpgsqlRangeTypeMapping.Default.Clone(
    comparer: new ValueComparer<NpgsqlRange<DateTime>>(
        (NpgsqlRange<DateTime> v1, NpgsqlRange<DateTime> v2) => v1.Equals(v2),
        (NpgsqlRange<DateTime> v) => v.GetHashCode(),
        (NpgsqlRange<DateTime> v) => v),
    keyComparer: new ValueComparer<NpgsqlRange<DateTime>>(
        (NpgsqlRange<DateTime> v1, NpgsqlRange<DateTime> v2) => v1.Equals(v2),
        (NpgsqlRange<DateTime> v) => v.GetHashCode(),
        (NpgsqlRange<DateTime> v) => v),
    providerValueComparer: new ValueComparer<NpgsqlRange<DateTime>>(
        (NpgsqlRange<DateTime> v1, NpgsqlRange<DateTime> v2) => v1.Equals(v2),
        (NpgsqlRange<DateTime> v) => v.GetHashCode(),
        (NpgsqlRange<DateTime> v) => v),
    mappingInfo: new RelationalTypeMappingInfo(
        storeTypeName: "tstzrange"),
    clrType: typeof(NpgsqlRange<DateTime>));
foo.TypeMapping = ((NpgsqlRangeTypeMapping)foo.TypeMapping).Clone(
    NpgsqlDbType.TimestampTzRange,
    NpgsqlTimestampTzTypeMapping.Default);

We may want to make this better in the future. For example, a type mapping could have an API which allows it to add a code fragment (or otherwise customize) its own initialization in the compiled model (a bit like how it has GenerateCodeLiteral, which generates a code fragment for instantiating a literal in C#).