refactor(const): Mark low level fns const

[a-kenji]

Mark functions of structs that suit themselves to be composed with multiple other widgets as const, namely:

• Block
• Layout
• Rect

Continuation of: #115

This allows with some limitations to share blocks, layouts, through many widgets and with some luck also to remove lazy_static! from some of those use cases.

[codecov]

Codecov Report

Merging #275 (65b1118) into main (bfcc550) will increase coverage by 0.22%.
The diff coverage is 86.51%.

❗ Current head 65b1118 differs from pull request most recent head 084bf41. Consider uploading reports for the commit 084bf41 to get more accurate results

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
+ Coverage   82.50%   82.73%   +0.22%     
==========================================
  Files          36       36              
  Lines        7301     7360      +59     
==========================================
+ Hits         6024     6089      +65     
+ Misses       1277     1271       -6     

Impacted Files	Coverage Δ
src/widgets/block.rs	84.15% <77.77%> (+0.85%)	⬆️
src/layout.rs	90.07% <94.87%> (+2.34%)	⬆️
src/style.rs	85.65% <100.00%> (+0.31%)	⬆️

[joshka]

Can you please add unit tests? These help document the intent of this change and prevent the accidental removal of the const keyword later. These can be simple one liners that check that we can use these in a constant context. Something like:

fn layout_can_be_const() {
  const layout: Layout = Layout::default()
    .margin(1)
    .horizontal_margin(1)
    ...
// or 
  const layout: Layout = Layout::default().margin(1);
  const layout: Layout = Layout::default().horizontal_margin(1);
  ...
}

For a commit message that targets the changelog, this is not a refactor as it really enables some extra functionality (the ability to use the layout and block in a constant context. So perhaps:

feat(misc): allow layout and block to be used in constant context

Block can now be used like:
`const BORDER_BLOCK: Block = Block::default().borders(...`
Layout can now be used like:
`const MAIN_LAYOUT: Layout = Layout::default().margin(...`

[a-kenji]

@joshka, thank you for the review!

For a commit message that targets the changelog, this is not a refactor as it really enables some extra functionality (the ability to use the layout and block in a constant context.

Agreed! I went for consistency here initially, but I changed the commit message now.

I added unit tests in the vein you were suggesting.
I also added the same unit tests for the const Style changes, should I keep them in this commit or split them into it's own commit?

[joshka]

I rebased on main and rewrote the commit message a little.
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