Reduce allocations in pem::encode

[alex]

Shows ~45% performance improvement

Before:

pem::encode             time:   [1.0654 µs 1.0674 µs 1.0696 µs]                         
Found 2 outliers among 100 measurements (2.00%)
  1 (1.00%) high mild
  1 (1.00%) high severe

pem::encode_many        time:   [2.2387 µs 2.2488 µs 2.2585 µs]                              

After:

pem::encode             time:   [597.59 ns 603.58 ns 609.70 ns]                         
                        change: [-44.590% -44.219% -43.829%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 19 outliers among 100 measurements (19.00%)
  2 (2.00%) high mild
  17 (17.00%) high severe

pem::encode_many        time:   [1.2678 µs 1.2723 µs 1.2771 µs]                              
                        change: [-43.377% -43.152% -42.931%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 7 outliers among 100 measurements (7.00%)
  6 (6.00%) high mild
  1 (1.00%) high severe

[jcreekmore]

Huh. I have literally never used core::fmt::Write explicitly before. I guess you learn something new every day. It makes complete sense why this would reduce allocations by roughly half, though.

[jcreekmore]

Released 3.0.1