Allow std::optional::and_then, std::optional::or_else, and std::optional::transform

[Daniel Cheng]

This is currently specifically banned in https://chromium.googlesource.com/chromium/src/+/HEAD/styleguide/c++/c++-features.md#Monadic-operations-for-std_optional-tbd; I'd like to propose allowing it by default.

For future C++ updates, I'd also propose just default allowing new methods on already-allowed classes when they don't also introduce externally-observable dependencies on banned or TBD library types.

Daniel

[Nico Weber]

lgtm

Part of the reason for the policy is that the TBD->Propose->Allow flow:

1. Raises awareness of new features

2. Makes sure the new features actually work, as allowing something entails adding a use of the thing in the same CL that allows it

So it has some overhead, but maybe that's worth it 🙃

Batching a bunch of these together definitely makes sense to me, though!

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