Add bytevector support

[mattwparas]

See bytevectors here: https://small.r7rs.org/attachment/r7rs.pdf

These could either be implemented as an enum variant in SteelVal or as just an opaque struct.

[EDToaster]

I see #169 implements bytevector. What else is needed to complete the implementation?

[mattwparas]

I haven't done an audit of the spec, but I believe there isn't support at the reader level for byte vectors yet, so doing the short hand byte vector syntax isn't possible at the moment