allow invalid UTF-8 string literals, deprecate b"..."

[StefanKarpinski]

also print invalid UTF-8 characters correctly

related: #25072

[StefanKarpinski]

This is ready to go. After the first commit, b"..." is no longer any more expressive than normal string literals, and we can get rid of cases where we have written String(b"...") to construct string literals containing invalid UTF-8, since they can now just be written as "..." directly. The additional decision is whether to deprecate b"abc" as a shorthand for Vector{UInt8}("abc") or keep it.

[quinnj]

I would vote to keep it; it can be handy sometimes.

[StefanKarpinski]

Ok, I can revert the deprecation and just keep the String(b"...") => "..." changes.

[StefanKarpinski]

This is pretty bad though (pointed out by @Keno):

julia> function f(p)
           v = b"123"
           p && (v[1] = 'x')
           return v
       end
f (generic function with 1 method)

julia> f(false)
3-element Array{UInt8,1}:
 0x31
 0x32
 0x33

julia> f(true)
3-element Array{UInt8,1}:
 0x78
 0x32
 0x33

julia> f(false)
3-element Array{UInt8,1}:
 0x78
 0x32
 0x33

We should consider changing this while we're at it.

[StefanKarpinski]

Triage resolution: make b"..." create a new vector each time instead of the same one every time.