Replace scanner's inefficient implementation

[Paalon]

Current scanner uses the implementation of f1 which is inefficient.

using BenchmarkTools

n = rand(10:20, 100);
css = rand.(Char, n);
ss = String.(css);

function f1(ss::Vector{String})
    v = Any[]
    for s in ss
        push!(v, s)
    end
    string(v...)
end

function f2(ss::Vector{String})
    v = String[]
    for s in ss
        push!(v, s)
    end
    join(v)
end

function f3(ss::Vector{String})
    io = IOBuffer()
    for s in ss
        write(io, s)
    end
    String(take!(io))
end

function f4(ss::Vector{String})
    result = ""
    for s in ss
        result *= s
    end
    result
end

@btime f1($ss);
# 11.076 μs (5 allocations: 7.72 KiB)
@btime f2($ss);
# 5.659 μs (12 allocations: 17.71 KiB)
@btime f3($ss);
# 3.711 μs (8 allocations: 11.63 KiB)
@btime f4($ss);
# 22.196 μs (100 allocations: 296.73 KiB)

We need to replace it to f3.