Support bytes as a data type

[relud]

fixes #81

[acmiyaguchi]

This looks good, it's missing an integration test under tests/resources/translate/atomic.json. Could you add the following test?

    {
      "description": [
        "Test that strings can be casted into byte strings"
      ],
      "name": "test_bytes_format",
      "compatible": true,
      "test": {
        "avro": {
          "type": "bytes"
        },
        "bigquery": [
          {
            "mode": "REQUIRED",
            "name": "root",
            "type": "BYTES"
          }
        ],
        "json": {
          "format": "bytes",
          "type": "string"
        }
      }
    }

With that, r+.

[acmiyaguchi]

Also, here's another thing that could be added to this PR, but doesn't have to.

jsonschema-transpiler/src/ast.rs

Lines 137 to 145 in 536c6fd

	(Type::Atom(left), Type::Atom(right)) => {
	let atom = match (left, right) {
	(Atom::Boolean, Atom::Boolean) => Atom::Boolean,
	(Atom::Integer, Atom::Integer) => Atom::Integer,
	(Atom::Number, Atom::Number)
	| (Atom::Integer, Atom::Number)
	| (Atom::Number, Atom::Integer) => Atom::Number,
	(Atom::String, Atom::String) => Atom::String,
	(lhs, rhs) => {

The union of a string and bytes should probably be bytes, for now it'll be dropped. Your thoughts?