Add support for SafeByte and SafeBytes

[arjunmahishi]

Currently, there is no direct way to write a safe byte or a byte slice to a SafeWriter. They have to be type-casted to either a rune (for byte) or a string (for []byte). It would be more efficient to add the types (and helpers) SafeByte and SafeBytes.

Also, UnsafeByte and UnsafeBytes already exist. So, introducing SafeByte and SafeBytes would also make the API more consistent.

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This change is 

[cockroach-teamcity]

All committers have signed the CLA.

[dhartunian]

LGTM

You probably want to add a tag with the next release version so we can bump in CRDB. Given this is a backwards compatible change, I think v1.1.6 is fine.

[arjunmahishi]

@dhartunian TFTR! Re: backward compatibility, two new methods are added to type SafeWriter interface. These methods are implemented in all types (within this package) that were conforming to this interface. But this is an exported interface. Would this still be considered backward compatible?

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Edit: I think v1.1.6 should be fine since there isn't really a usecase for an external implementation of this interface. Plus, Go modules should ensure that there is no accidental upgrade.