Make excpetions optional depending on build-time feature checks.

[chuim]

Replaces all try/catch/throw keyword usage with a set of macros that are
appropriately defined in config.hpp based on the status of the
cxx_exceptions compiler feature.

Reported in issue #160.

[codecov-commenter]

Codecov Report

Merging #162 into master will decrease coverage by 0.02%.
The diff coverage is 80.28%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   94.15%   94.12%   -0.03%     
==========================================
  Files         103      103              
  Lines       10002    10011       +9     
==========================================
+ Hits         9417     9423       +6     
- Misses        585      588       +3     

Impacted Files	Coverage Δ
immer/detail/util.hpp	78.43% <0.00%> (ø)
immer/heap/gc_heap.hpp	81.81% <0.00%> (ø)
immer/heap/malloc_heap.hpp	83.33% <0.00%> (ø)
test/map/generic.ipp	100.00% <ø> (ø)
test/vector/generic.ipp	99.63% <ø> (ø)
immer/detail/arrays/with_capacity.hpp	88.72% <9.09%> (ø)
immer/detail/arrays/no_capacity.hpp	89.85% <20.00%> (ø)
immer/detail/arrays/node.hpp	91.89% <50.00%> (ø)
immer/detail/rbts/node.hpp	95.80% <81.25%> (ø)
immer/detail/hamts/node.hpp	96.86% <88.46%> (ø)
... and 9 more

---

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[arximboldi]

Overall LGTM. Have you run clang-format on it?

[arximboldi]

Cool, thanks! I think the last incarnation is acceptable. Merging!