decoder: Preallocate .buf capacity when we know (approximate) size of output to it

This reduces allocations a bit:

    name      old time/op    new time/op    delta
    Speed-4      383ns ± 4%     378ns ± 0%    ~     (p=0.302 n=5+5)
    Decode-4    72.1µs ± 1%    70.1µs ± 0%  -2.76%  (p=0.008 n=5+5)
    Encode-4    16.5µs ± 0%    16.6µs ± 0%    ~     (p=0.095 n=5+5)

    name      old alloc/op   new alloc/op   delta
    Speed-4       280B ± 0%      280B ± 0%    ~     (all equal)
    Decode-4    35.7kB ± 0%    32.3kB ± 0%  -9.72%  (p=0.008 n=5+5)
    Encode-4    6.54kB ± 0%    6.54kB ± 0%    ~     (all equal)

    name      old allocs/op  new allocs/op  delta
    Speed-4       8.00 ± 0%      8.00 ± 0%    ~     (all equal)
    Decode-4       429 ± 0%       428 ± 0%  -0.23%  (p=0.008 n=5+5)
    Encode-4       297 ± 0%       297 ± 0%    ~     (all equal)

ogorek.go

@@ -536,6 +536,7 @@ func (d *Decoder) loadBinString() error {
 	v := binary.LittleEndian.Uint32(b[:])
 
 	d.buf.Reset()
+	d.buf.Grow(int(v))
 	_, err = io.CopyN(&d.buf, d.r, int64(v))
 	if err != nil {
 		return err
@@ -551,6 +552,7 @@ func (d *Decoder) loadShortBinString() error {
 	}
 
 	d.buf.Reset()
+	d.buf.Grow(int(b))
 	_, err = io.CopyN(&d.buf, d.r, int64(b))
 	if err != nil {
 		return err
@@ -568,6 +570,7 @@ func (d *Decoder) loadUnicode() error {
 	sline := string(line)
 
 	d.buf.Reset()
+	d.buf.Grow(len(line)) // approximation
 
 	for len(sline) > 0 {
 		var r rune
@@ -925,6 +928,7 @@ func (d *Decoder) loadShortBinUnicode() error {
 	}
 
 	d.buf.Reset()
+	d.buf.Grow(int(b))
 	_, err = io.CopyN(&d.buf, d.r, int64(b))
 	if err != nil {
 		return err