Encode constants with `const`, not `#define`

[ollie-etl]

The issue is created to record the const aspect of #156. The motivations remain as that issue.

Some points on the effect of using const in place of #define

• Symbols for the constants will be included in library. This increases the size of the library.
• This is only of concern in systems with both using dynamic linkage and with tight space constraints.
• There are 640 #defines (in total) in the codebase. This should bound the upper limit on the effect on size to a few kB.

Many of the constants could better be represented as enums, for increased type safety. This has a larger impact on codebases using libmetal, so such a refactoring should be considered as a seperate issue.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.