DRTVWR-589: Collect int table keys in vector<int>, not set<int>.

Given that we at least have a possibility of determining the length of a Lua
table in advance, we might be able to populate a vector of keys with a single
initial allocation. Even if Lua reports the length incorrectly,
vector::push_back() is one of the bread-and-butter operations of the library,
optimized to the extent possible. Inserting elements into a set seems more
likely to incur allocations.

Of course, we must then sort() the vector to determine its largest key value.

Also document the requirement that we use a Lua runtime compiled for C++, that
is, compiled to raise errors by C++ exceptions rather than by longjmp(). We
rely on temporary stack objects being properly destroyed even if errors are
raised.

Conventionally, with lua_tomumble(L, index), 'index' refers to the stack index
of the Lua object being converted to C++. For a Lua table, talk about table
keys rather than table indexes to avoid confusing the maintainer.

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