Simplify converting to `Vector{UInt8}`

[nickrobinson251]

• Saves an allocation compared to the current Base method which first calls String(s)
• related to #5
• covered by existing tests

before:

julia> s = String31("abcd_efgh")
"abcd_efgh"

julia> @time Vector{UInt8}(s);
  0.000012 seconds (2 allocations: 96 bytes)

now:

julia> @time Vector{UInt8}(s);
  0.000011 seconds (1 allocation: 64 bytes)