ruby: decode - add "=\xF3rl"
ruby: speedup the encoder; branch predict; alloc 10k chunks; silence some warnings

* speed up the encoder with 30% (parse 30mb complex object in 0.8s - down from 1.1s)
* add some branch prediction
* bump the allocation to 10k chunks (was 512b)
* speedup s_append_varint, just prepare the max varint size and dont
  check size during construction [ hijacked from perl/srl_encoder.c ]

some results:

hash with 100 random keys/values:
before:
srl-e    137730.0 (±8.5%) i/s -     686340 in   5.019890s
srl-d     36225.7 (±3.9%) i/s -     181980 in   5.031649s
msg-e    237257.6 (±2.9%) i/s -    1203520 in   5.077060s
msg-d     36639.7 (±4.9%) i/s -     184552 in   5.050058s
jsn-e     22573.7 (±2.7%) i/s -     114375 in   5.070675s
jsn-d     16180.9 (±2.7%) i/s -      81833 in   5.061138s

after: 44% bump
srl-e    249645.9 (±4.1%) i/s -    1261113 in   5.061096s
srl-d     39008.8 (±2.4%) i/s -     197796 in   5.073712s
msg-e    242311.7 (±3.2%) i/s -    1227240 in   5.070461s
msg-d     39380.7 (±2.9%) i/s -     199854 in   5.079275s
jsn-e     22738.2 (±3.2%) i/s -     114972 in   5.062180s
jsn-d     16982.6 (±4.5%) i/s -      85425 in   5.040897s

small array: [1, {"a"=>"b"}, 3, "aaa"]"
before:
srl-e   1089028.9 (±5.2%) i/s -    5464800 in   5.033625s
srl-d    568083.3 (±4.5%) i/s -    2836620 in   5.003850s
msg-e    855232.7 (±6.1%) i/s -    4282551 in   5.029888s
msg-d    387226.1 (±3.6%) i/s -    1956555 in   5.059796s
jsn-e    233556.3 (±4.1%) i/s -    1174632 in   5.038752s
jsn-d    176396.1 (±3.2%) i/s -     883646 in   5.014920s

after: 13% bump
srl-e   1255427.1 (±3.7%) i/s -    6270680 in   5.002321s
srl-d    617622.3 (±7.3%) i/s -    3082872 in   5.022283s
msg-e    915127.3 (±4.7%) i/s -    4581748 in   5.019316s
msg-d    412708.0 (±8.3%) i/s -    2075543 in   5.069298s
jsn-e    234972.9 (±6.1%) i/s -    1185024 in   5.063820s
jsn-d    185578.3 (±2.8%) i/s -     930605 in   5.018926s