Rename type macros in UpperCamelCase.

[Anders429]

Fixes #141.

This renames the registry! and views! macros to Registry! and Views!, respectively. It also ensures that only the macros are reexported, while the traits are kept private.

This ultimately helps with clarity over which macros are for types and which macros are not. While this isn't officially part of the Rust API guidelines, since it has passed a final comment period and is currently in use in big crates like syn (see syn::Token!, I think it's the right direction to go.

[codecov-commenter]

Codecov Report

Merging #147 (aa1161e) into dev (9d2168d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #147   +/-   ##
=======================================
  Coverage   94.81%   94.81%           
=======================================
  Files          73       73           
  Lines       10222    10222           
=======================================
  Hits         9692     9692           
  Misses        530      530           

Impacted Files	Coverage Δ
src/archetype/impl_serde.rs	99.82% <ø> (ø)
src/archetypes/impl_serde.rs	98.14% <ø> (ø)
src/archetypes/mod.rs	99.04% <ø> (ø)
src/entities/mod.rs	100.00% <ø> (ø)
src/entity/allocator/impl_serde.rs	98.76% <ø> (ø)
src/entity/allocator/location.rs	53.84% <ø> (ø)
src/entity/allocator/slot.rs	82.85% <ø> (ø)
src/entity/identifier/mod.rs	100.00% <ø> (ø)
src/entity/mod.rs	100.00% <ø> (ø)
src/lib.rs	100.00% <ø> (ø)
... and 24 more

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