- Use the Windows type `HMODULE` on Windows, and define a local alias to
it on non-Windows. This makes the code slightly clearer about the
nature of the value `handle`.
- Move the `dlopen` emulation into the anonymous namespace to encourage
inlining, since the wrappers are trivial name/signature compatibility
functions.
- Move the error reporting out of the template function, so that it is
not redundantly instantiated for each type used with the template.
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