By not materializing the lower cased tags ahead of time, we
save allocations at the cost of worsened performance when comparing
two tags.

Since there are rarely many tags, and commonly they are not
compared even if present, this is almost always a win. The new
implementation also improves the robustness of the code
responsible for handling tags in a case-insensitive manner.

This change also changes it so that test case macros using a
class name can have same name **and** tags as long as the
used class name differs.

Closes #1915
Closes #1999

Previous splitting of catch_common.hpp left it containing only one
actual thing, which is the `SourceLineInfo` type. Given that, there
is no reason to keep the old name.

Also found out that it was included in some places for no reason
(primarily Matchers).

This should also be done for test files, but that has lower priority.

This commit also strips the old copyright comment header in touched
files, as those will also be replaced with a more standardized and
machine-friendly version.

Doing some benchmarking with ClangBuildAnalyzer suggests that
compiling Catch2's `SelfTest` spends 10% of the time instantiating
`std::unique_ptr` for some interface types required for registering
and running tests.

The lesser compilation overhead of `Catch::Detail::unique_ptr` should
significantly reduce that time.

The compiled implementation was also changed to use the custom impl,
to avoid having to convert between using `std::unique_ptr` and
`Catch::Detail::unique_ptr`. This will likely also improve the compile
times of the implementation, but that is less important than improving
compilation times of the user's TUs with tests.