mime: do a pre-allocation in encodeWord

The preallocated memory size is comparable to bytes.Buffer bootstraping
array length (bytes.Buffer was used before rewrite to strings.Builder).

Without preallocation, encodeWord does more than one allocation for
almost any possible input.

The regression happens because bytes.Buffer did a 80-bytes allocation
at the beginning of encodeWord while strings.Builder did several
smaller allocations (started with cap=0).

Comparison with reported regression:

  name           old time/op    new time/op    delta
  QEncodeWord-4     781ns ± 1%     593ns ± 1%  -24.08%  (p=0.008 n=5+5)

  name           old alloc/op   new alloc/op   delta
  QEncodeWord-4      152B ± 0%       80B ± 0%  -47.37%  (p=0.008 n=5+5)

  name           old allocs/op  new allocs/op  delta
  QEncodeWord-4      5.00 ± 0%      2.00 ± 0%  -60.00%  (p=0.008 n=5+5)

Comparison with buffer solution (like before strings.Builder, but
without sync pool for buffer re-using):

  name           old time/op    new time/op    delta
  QEncodeWord-4     595ns ± 1%     593ns ± 1%     ~     (p=0.460 n=5+5)

  name           old alloc/op   new alloc/op   delta
  QEncodeWord-4      160B ± 0%       80B ± 0%  -50.00%  (p=0.008 n=5+5)

  name           old allocs/op  new allocs/op  delta
  QEncodeWord-4      2.00 ± 0%      2.00 ± 0%     ~     (all equal)

We avoid allocation in buf.String(), as expected.

Fixes #25379

Change-Id: I19763f0e593a27390c1a549b86ce6507b489046b
Reviewed-on: https://go-review.googlesource.com/113235


Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>