Improve unsupported coercion performance by optionally caching error

[ivoanjo]

As identified in https://github.com/solnic/coercible/issues/16 and https://github.com/jruby/jruby/issues/4540, coercible's use of errors to signal unsupported coercions has a measurable impact on performance, especially when using JRuby.

To workaround this problem, this PR adds the option (controlled by the setting the COERCIBLE_CACHE_UNSUPPORTED_COERCION_ERROR environment variable to '1', off by default) to reuse a cached error object, rather than raising a new instance every time. This saves a big slice of the cost of using errors, but trades it off with having a useless backtrace -- all UnsupportedCoercion errors will share the same backtrace.

It also includes a benchmark that can be used to measure the impact of this change. Here are some results of the benchmark on my local machine (Core i7-6500U, Ubuntu 17.04):

• ruby 2.4.1p111 (2017-03-22 revision 58053) [x86_64-linux]:

  cached_unsupported_coercion_error
    559.584k (± 8.3%) i/s -     16.664M in  30.057606s
  noncached_unsupported_coercion_error
    287.123k (±13.9%) i/s -      8.402M in  30.018748s
  
  Comparison:
    cached_unsupported_coercion_error:   559583.7 i/s
    noncached_unsupported_coercion_error:   287123.4 i/s - 1.95x  slower

• jruby 9.1.7.0 (2.3.1) 2017-01-11 68056ae Java HotSpot(TM) 64-Bit Server VM 25.131-b11 on 1.8.0_131-b11 +indy +jit [linux-x86_64]:

  cached_unsupported_coercion_error
    797.391k (± 9.1%) i/s -     23.619M in  29.979835s
  noncached_unsupported_coercion_error
    589.736k (± 9.0%) i/s -     17.477M in  29.996031s
  
  Comparison:
    cached_unsupported_coercion_error:   797390.8 i/s
    noncached_unsupported_coercion_error:   589735.9 i/s - 1.35x  slower

• jruby 9.1.12.0 (2.3.3) 2017-06-15 33c6439 Java HotSpot(TM) 64-Bit Server VM 25.131-b11 on 1.8.0_131-b11 +indy +jit [linux-x86_64]

  cached_unsupported_coercion_error
    843.364k (± 8.4%) i/s -     24.977M in  29.973919s
  noncached_unsupported_coercion_error
    5.757k (±10.5%) i/s -    169.785k in  30.029482s
  
  Comparison:
    cached_unsupported_coercion_error:   843363.9 i/s
    noncached_unsupported_coercion_error:     5757.1 i/s - 146.49x  slower

  (The difference here to 9.1.7.0 is due to the "regression" in https://github.com/jruby/jruby/issues/4540)