Allocation improvements

[Anarh2404]

Proposed Changes

Ported some changes from the #734
Made ContantHeader and Method readers/writers structs

Types of Changes

• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)

Checklist

• I have read the CONTRIBUTING.md document
• I have signed the CA (see https://cla.pivotal.io/sign/rabbitmq)
• All tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in related repositories

Further Comments

Before

After

[michaelklishin]

@Anarh2404 thank you. Now that #732 is in, are we expected to merge this PR next or there can be some overlap/competition with #706? We really could use some guidance as while all these allocation improvement PRs are appreciated, they tend to compete and not collaborate. We would really appreciate a single unified effort from you and @stebet instead of competing PRs.

[Anarh2404]

@michaelklishin this PR complements #732 and does not compete with #706

[michaelklishin]

Great, thanks for clarifying.

[stebet]

Looks good to me :)