Procedural API for SumType

[pbackus]

This PR introduces three new convenience functions that allow code using SumType to be written in a more procedural style (as opposed to the functional style of match).

• has!T checks whether a SumType contains a value of type T.
• get!T accesses a SumType's value if it has type T, or asserts if it does not.
• tryGet!T is like get!T, but throws an exception instead of asserting on failure.

To make room for get!T, the existing SumType.get method is renamed to getByIndex.

CC @atilaneves since this introduces new public symbols
CC @jmdavis who requested this in the community Discord

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#10650"

[LightBender]

Does this need a changelog entry or a corresponding GH issue? Beyond that I approve of this.

[pbackus]

Added a changelog entry.

[pbackus]

CI failure appears to be a bug in the ImportC test suite:

src\importc_compare.d-mixin-134(134): Error: C preprocess command C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.43.34808\bin\HostX64\x64\cl.exe failed for file src\importc_includes.c, exit status 2

[...]

C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.43.34808\include\vcruntime_c11_stdatomic.h(16): fatal error C1189: #error: "C atomics require C11 or later"

[pbackus]

This has been sitting with the 72h no objection -> merge label for 4 days, so I'm clicking merge.