More configurable TOML serialization

Right now, the toml serializer generates only top level properties, and inline tables where necessary (i.e. where arrays are used):

```toml
abc.foo = 1
abc.bar = 2
abc.xyz = [{foo = 1, bar = 2}]
```

There are special cases where we *could* generate normal toml tables. The example above can be expressed as:

```toml
[abc]
foo = 1
bar = 2

[[abc.xyz]]
foo = 1
bar = 2
```

This format will in many cases be more readable. However, there are problems with non-inline tables that prevent us from emitting them in a streaming generator:

- Once a table is started, it is impossible to go back to the parent or root table. If an object has scalar properties that we only see after we started a subtable, we have a problem.
- Array tables only work with arrays of objects. If an array is heterogeneous, but we've already started emitting it as an array table, we have a problem.

These problems can only be solved with knowledge of the tree being serialized, and perhaps even by influencing property order so that scalars always come first. However, because I expect serialization to toml to be fairly niche – it is a configuration format meant to be written by humans, after all – it is not worth adding machinery to databind to implement this.

Another question to consider: What even *is* the best representation of an object? A short inline object may be more readable than starting a new table that includes the entire path to that object.

A possible solution to this problem would be to add API to the TomlGenerator to specifically start a table. One approach would be to create overloads for `writeStartObject` and `writeStartArray` that allow forcing generation as a table. If data is then passed that cannot be represented as a table (the bullet points above), we would error. While we're at it, could also allow emitting comments with additional methods.

We could also inspect annotations on the `forValue` passed to `writeStartObject`, though this seems like misuse of that parameter, so not a good idea.