Use String.build instead of IO::Memory.

[didactic-drunk]

New faster implementation of Base32.encode.
Correctly handle NUL bytes.
Add NUL spec.

[didactic-drunk]

New benchmark results:

• encode short 4.20M (238.16ns) (±18.68%) 176B/op fastest
• encode long 80.06k ( 12.49µs) (±13.37%) 6.11kB/op 52.45× slower

Old benchmark results:

• encode short 3.32M (301.47ns) (± 3.11%) 272B/op fastest
• encode long 44.99k ( 22.23µs) (± 2.54%) 15.7kB/op 73.73× slower

Code:

require "benchmark"
require "./src/base32"

bytes1 = "foobarbaz".to_slice
bytes2 = Bytes.new 1024
Benchmark.ips do |bm|
  bm.report "encode short" do
    Base32.encode bytes1, false
  end

  bm.report "encode long" do
    Base32.encode bytes2, false
  end
end