Improve code readability

[turbolent]

Depends on #78

Description

While trying to understand the contract, my OCD got the best of me, sorry.

Try to improve the readability of the code by e.g.:

• Improving argument labels (avoid abbreviations)
• Adding whitespace, e.g. between declarations, for single-line if statements
• Chopping down parameter and argument lists onto separate lines when the are two long
• Realigning comments, use semantic line breaks
• Fixing document comments vs normal comments
• Using switch statement instead of if statement when working with enum cases to get exhaustiveness checking
• Replacing nil checks and force unwrapping (if v != nil { ... v! }) with if let statements or nil coalescing (??)
• Removing redundant type annotations for variable declarations when they can be inferred

Best viewed by ignoring whitespace changes: https://github.com/onflow/FlowCreditMarket/pull/79/files?w=1

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For contributor use:

• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the standards mentioned here.
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels