AVRO-1517: [Perl] Encode UTF-8 strings as bytes

[jjatria]

From John Karp's original description of the original issue at https://issues.apache.org/jira/browse/AVRO-1517:

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.

Verifying this change

This change extended t/01_schema.t to add tests for the encoding of different kind of strings (including upgraded ones) as type 'byte' and 'fixed'.

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

Change was documented in the change log.