Implement Support for no_std

[CryZe]

This adds the features std and alloc to the crate. The std feature is activated by default. This would bump the minimum supported version to 1.34.0, but only due to the byteorder removal (which apparently is also happening in #110).

Supersedes #62

[marshallpierce]

This is great, thank you.

A few thoughts:

• Can you add a bit of docs to the README explaining when to use the features?
• I'm new to the no_std world so maybe there's something obvious I'm missing, but alloc was stabilized in 1.36.0 -- wouldn't that need to be the minimum version here?
• What can we do CI-wise to ensure that we don't break things for a no-std user?

[CryZe]

alloc is only used when the std feature is deactivated and the alloc feature is. So 1.34.0 can still be the minimum version (but lower is also possible, we only use it for the byte reading stuff).

For CI we can compile to thumbv6m-none-eabi, which doesn't have std. I'll look into making it compile to that. However we can't run the tests then.

I'll put a section into the README.

[CryZe]

This should be good to go now.

[codecov-io]

Codecov Report

Merging #111 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #111      +/-   ##
=========================================
+ Coverage   95.88%   95.9%   +0.02%     
=========================================
  Files          12      12              
  Lines        1457    1490      +33     
=========================================
+ Hits         1397    1429      +32     
- Misses         60      61       +1

Impacted Files	Coverage Δ
src/lib.rs	92.3% <ø> (ø)	⬆️
src/chunked_encoder.rs	98.78% <ø> (ø)	⬆️
src/display.rs	100% <ø> (ø)	⬆️
src/encode.rs	98.05% <100%> (+0.02%)	⬆️
src/decode.rs	94.42% <100%> (+0.02%)	⬆️
tests/decode.rs	99.27% <0%> (-0.73%)	⬇️
src/write/encoder_tests.rs	94.66% <0%> (+0.03%)	⬆️
tests/tests.rs	98.85% <0%> (+0.04%)	⬆️
tests/encode.rs	97.56% <0%> (+0.68%)	⬆️

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[marshallpierce]

Great, thank you!