Improve SpreadLayout and StorageLayout derive macro hygiene

[Robbepop]

PR and side-work resulting from my work on https://github.com/paritytech/ink/pull/665/commits.

[codecov-commenter]

Codecov Report

Merging #926 (2672dd2) into master (4155ebe) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #926      +/-   ##
==========================================
+ Coverage   84.24%   84.49%   +0.24%     
==========================================
  Files         172      158      -14     
  Lines        7945     6551    -1394     
==========================================
- Hits         6693     5535    -1158     
+ Misses       1252     1016     -236     

Impacted Files	Coverage Δ
crates/storage/derive/src/spread_layout.rs	98.48% <ø> (ø)
crates/storage/derive/src/storage_layout.rs	94.11% <100.00%> (ø)
crates/lang/ir/src/ir/item_impl/mod.rs	73.58% <0.00%> (-17.80%)	⬇️
crates/lang/codegen/src/generator/metadata.rs	90.90% <0.00%> (-6.70%)	⬇️
crates/lang/ir/src/ir/item_impl/impl_item.rs	51.35% <0.00%> (-6.10%)	⬇️
crates/lang/ir/src/ir/idents_lint.rs	68.42% <0.00%> (-3.01%)	⬇️
crates/lang/ir/src/ir/config.rs	88.88% <0.00%> (-2.61%)	⬇️
crates/lang/ir/src/ir/trait_def.rs	92.89% <0.00%> (-0.21%)	⬇️
crates/metadata/src/lib.rs	0.00% <0.00%> (ø)
crates/primitives/src/key_ptr.rs	100.00% <0.00%> (ø)
... and 34 more

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