avro

AVRO-1517: [Perl] Encode UTF-8 strings as bytes (#2979)

From John Karp's original description of [the issue]:

> By default in Perl, a string is a sequence of bytes, values 0-255.
> However, if a Unicode character is included that cannot be represented
> with a single byte, the string gets 'upgraded' to a non-byte-based
> Unicode string allowing ordinals outside that range. When string
> operations are done with byte and non-byte Unicode strings, the result
> is always non-byte, with the byte string first 'upgraded'. Upgrading
> consists of utf8 encoding and setting a utf8 flag on the string. ('utf8'
> is a variant of UTF-8 used by Perl)
>
> The Perl Avro API is accepting these Unicode strings as-is for the
> 'bytes' type. This is a problem because
>
>   1. values >255 are not valid as bytes, and any encoding is their job
>
>   2. As Avro assembles the serialized data, Perl 'upgrades' all the data,
>      having the effect of utf8 encoding our serialized binary data.
>
> The correct behavior is for the Avro Perl API is to attempt to downgrade
> the string, and if this fails because it contained values >255 then to
> raise an error. (The behavior of 'string' won't change, it will still
> take Unicode strings as expected.)

This change, based on the one submitted for that ticket, adds these
behaviours and tests to exercise them.

[the issue]: https://issues.apache.org/jira/browse/AVRO-1517