use built-in 'Default' trait

[danieleades]

• use the built-in Default trait for constructing structs with no arguments
• use idiomatic new method for constructing structs with arguments

[danieleades]

i've updated this PR in light of the fact that the other breaking changes have been merged.

[danieleades]

@dwrensha i've refreshed this PR, and deprecated the existing methods rather than removing them outright

[codecov]

Codecov Report

Attention: 21 lines in your changes are missing coverage. Please review.

Comparison is base (5609a45) 51.88% compared to head (868c67d) 51.82%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
capnp/src/private/layout.rs	34.61%	17 Missing ⚠️
capnp/src/message.rs	75.00%	2 Missing ⚠️
capnp-rpc/src/rpc.rs	0.00%	1 Missing ⚠️
capnpc/test/test.rs	98.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #340      +/-   ##
==========================================
- Coverage   51.88%   51.82%   -0.06%     
==========================================
  Files          69       69              
  Lines       34113    34116       +3     
==========================================
- Hits        17698    17680      -18     
- Misses      16415    16436      +21     

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[danieleades]

@dwrensha is this still of interest?

[dwrensha]

I added impl Default for message::Builder<HeapAllocator> in 2018225 because that seems potentially helpful if users are defining their own structs that include a message::Builder and they want to have impl Default on those structs. I don't want to remove message::Builder::new_default() because requiring users to find a trait method seems like a potential source of confusion (particularly in situations where the standard prelude is not included). The other changes here are to internal details and don't seem like an improvement to me.