build: Improvements to symbol visibility logic on Windows (attempt 3)

[hebasto]

Previous attempts:

• build: Use SECP256K1_STATICLIB macro instead of warning suppressions #1346
• build: Improvements to symbol visibility logic on Windows #1362

The result is as follows:

1. Simple, concise and extensively documented code.
2. Explicitly documented use cases with no ambiguities.
3. No workarounds for linker warnings.
4. Solves one item in build: Meta-issue for follow-ups to initial CMake merge (#1113) #1235.