implement ToJson/FromJson traits for types defined in core

[Yoorkin]

[qazxcdswe123]

• [x] float
• [x] fixedarray
• [x] bigint
• [x] byte
• [ ] bytes
• [x] option
• [x] result
• [x] unit

[qazxcdswe123]

Should byte and bytes be base64 encoded string in json? @bobzhang @Yoorkin This is what protobuf does.

ref:

bytes	base64 string	"YWJjMTIzIT8kKiYoKSctPUB+"	JSON value will be the data encoded as a string using standard base64 encoding with paddings. Either standard or URL-safe base64 encoding with/without paddings are accepted.

[hackwaly]

I think bytes should encoded as byte[], which byte is number of 0-255.