### Context & Motivation
The error handling utilities in `//base/error.h` are very useful for
writing code with strong safety guarantees. While hardening the `Dump`
debug utilities (from review in #4866), I encountered a rough edge with
references and pointers. After a [brief Discord discussion in
#contributing-help](https://discord.com/channels/655572317891461132/1052653651895779359/1334675462877610038),
it was suggested that adding support for references to `ErrorOr` would
be a good candidate to move forward.
Using a reference type with the `ErrorOr` class (e.g. `ErrorOr<Node&>`)
produces two errors:
<ol>
<li><strong><code>variant can not have a reference type as an
alternative</code></strong>
<ul><li>From private field: <code>std::variant<Error, T>
val_;</code></li></ul>
</li>
<li><strong><code>'operator->' declared as a pointer to a
reference</code></strong>
<ul><li>From member function: <code>auto operator->() ->
T*</code></li></ul>
</li>
</ol>
### Changes
To support reference types, both errors are resolved:
1. `std::reference_wrapper` is conditionally used for storage when `T`
is a reference type
2. type trait aliases like `using ValueT = std::remove_reference_t<T>`
are used to produce compatible types for methods like `auto operator->()
-> ValueT*`
Calvin
dcfccd3187