Add to apply_test.cpp a collect() function that incrementally accumulates an
arbitrary number of arguments into a std::vector<std::string>. Construct a
std::array<std::string> to pass it, using VAPPLY().
Clarify in header comments that LL::apply() can't call a variadic function
with arguments of dynamic size: std::vector or LLSD. The compiler can deduce
how many arguments to pass to a function with a fixed argument list; it can
deduce how many arguments to pass to a variadic function with a fixed number
of arguments. But it can't compile a call to a variadic function with an
arguments data structure whose size can vary at runtime.
(cherry picked from commit ceed33396266b123896f7cfb9b90abdf240e1eec)
Make apply(function, std::array) and apply(function, std::vector) available
even when we borrow the C++17 implementation of apply(function, std::tuple).
Add apply(function, LLSD) with interpretations:
* isUndefined() is treated as an empty array, for calling a nullary function
* scalar LLSD is treated as a single-entry array, for calling a unary function
* isArray() converts function parameters using LLSDParam
* isMap() is an error.
Add unit tests for all flavors of LL::apply().
(cherry picked from commit 3006c24251c6259d00df9e0f4f66b8a617e6026d)
Andrey Kleshchev