Binary-read fixed-length String

[jling]

even simpler:

julia> v=UInt8[0x54,0x68,0x69,0x73]
4-element Vector{UInt8}:
 0x54
 0x68
 0x69
 0x73

julia> String(v)  # just return this
"This"

julia> v
UInt8[]

this is non-copy