Generates an Avro schema for `spec-k`.

`spec-k`: the key of the Spec to convert

Returns a tuple: [`::spec->avro-ref` `::avro-schema`].

The `::spec->avro-ref` map is updated in any calls that define Avro named
types, so that schemas are only defined on the first instance and then
referred to by fully qualified name.

NOTE: you shouldn't need to call this directly, but you can extend this
multimethod to implement schema generation for specs that aren't included
here.

See `abracad.avro/binary-encoded`.

See `abracad.avro/decode`.

Specs defined as non-empty, keyword-only sets are `::enum`s.

`::enum`s are used for defining a finite set of choices and are converted to
Avro `:enum`s. An example would be:

`(s/def ::color #{:red :green :blue})'

Processes a deserailized value `v` from Avro.

For example, it converts bytes to a `BigDecimal` for an Avro `:decimal`,
integers to `Instants` for `:timestamp-millis` and so on, according to the
Avro specification.

See `post-deserialize`.

NOTE: you shouldn't need to call this directly, but you can extend this
multimethod to implement value post-deserialization for specs that aren't
included here.

Prepares a value `v` for encoding to Avro.

For example, it converts `BigDecimal`s to bytes for Avro `:decimal`s,
`:timestamp-millis` to integers and so on, according to the Avro
specification.

See `pre-serialize`.

NOTE: you shouldn't need to call this directly, but you can extend this
multimethod to implement value pre-serialization for specs that aren't
included here.