support keyEncoding: binary

[kumavis]

if keys are buffers, they are coerced to utf8 strings. utf8 strings dont support arbitrary binary data (e.g. hashes). invalid utf8 byte sequences are replaced with the diamond-question mark character, potentially causing unintended key collisions. I also imagine the subsequent throw-catch ruins performance.

[kumavis]

Never sure how to do a proper perf test as compiler optimizations are so dependant on context but, seems like it takes an additional 18% to serialize the binary buffer

var bin = Buffer('e01022dceb8c3202', 'hex') // bin.length => 8
var str = Buffer('haaywurl', 'utf8')        // str.length => 8

bin.toString() //=> '�\u0010"���2\u0002'
str.toString() //=> 'haaywurl'

var iterations = 1000000
console.log( test(bin, iterations)[1]/iterations )
console.log( test(str, iterations)[1]/iterations )

function test(buf, iterations){

  var start = process.hrtime()
  for(var i=0; i<iterations; i++) {
    buf.toString()
  }
  return process.hrtime(start)

}

╭─{ } kumavis in ~/Development/Node/merkle-patricia-tree on (master✱) 
╰─± node buf-test.js
419.518481
356.506166
╭─{ } kumavis in ~/Development/Node/merkle-patricia-tree on (master✱) 
╰─± node                    
> 419.518481 / 356.506166
1.1767495796973115

[kumavis]

Hmm I think keyEncoding is a levelup thing, so not sure why i filed this here