Split encode/decode error policy

[cppden]

Provide separate control on error propagation for encoding and decoding.
Default is throw during encode but bool for encode to reduce sensitivity to DoS attacks (see benchmarks).

[cppden]

according to latest benchmarks exception error handling is faster in 100 (a hundred, Karl!) times. thus current plan is to have it by default and no reason for split enc/dec.

bool return (current defaults):

Benchmark               Time           CPU Iterations
------------------------------------------------------
BM_encode_ok         1107 ns       1105 ns     625296
BM_encode_fail        213 ns        212 ns    3298930
BM_decode_ok         1359 ns       1356 ns     512868
BM_decode_fail       1112 ns       1110 ns     622017

exceptions (future defaults)

Benchmark               Time           CPU Iterations
------------------------------------------------------
BM_encode_ok           15 ns         15 ns   47323241
BM_encode_fail       1904 ns       1903 ns     367928
BM_decode_ok           23 ns         23 ns   31101858
BM_decode_fail       3919 ns       3916 ns     176466