Split array heap flag into std and no std variants

[gregorygaines]

As a follow-up to #70, split the array heap flag into std and no std variants to support each environment.

[gregorygaines]

@Eisverygoodletter Can you take a look at this to make sure everything is 100%?

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.27%. Comparing base (06039c3) to head (389ceb5).

Files with missing lines	Patch %	Lines
...ating/bitfield/features/bitfield_struct_feature.rs	85.71%	1 Missing ⚠️
...ating/bitfield/features/common/generator_helper.rs	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   97.97%   98.27%   +0.30%     
==========================================
  Files          38       38              
  Lines        3008     3023      +15     
==========================================
+ Hits         2947     2971      +24     
+ Misses         61       52       -9     

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[Eisverygoodletter]

Seems good to me 👍

[Eisverygoodletter]

For the test for no-std + alloc you may have to extern crate alloc and then implement a dummy #[global_allocator] that does nothing for it to link and build.