feat: add support for reading free floating std::vector

[ioanaif]

No description provided.

[jpivarski]

Great! I went back and forth on the priority order; now it's

1. check for any globally- or per-file-defined Models
2. load the file's streamers and look for any Models the streamers might have defined
3. check for explicit STL classes, like std::vector<T> (where the T might have been defined by one of the streamers, so we had to check
4. give up and call it UnknownClass.

The regex is now

(std\s*::\s*)?(vector|map|set|string|bitset)

because we want exactly one of the STL class names to match.

But it looks good, the test does the right thing, and it's ready to merge.

[jpivarski]

Another alteration: this is similar to #1160, but that implementation checked for std::string in ReadOnlyKey.get. It has to be in a different place: since free-floating std::string has to return a str type and not a Model, ReadOnlyFile.class_named is too late. The other cases, std::vector, std::map, std::set, and std::bitset (which are the only ones with parameters; std::string wouldn't be followed by a <) return container types such as STLVector, STLMap, STLSet, and STLBitSet.

However, I could make the code that handled the #1160 case more symmetric. It can check for both string and std::string.

Now it's ready to merge.