Previously, any ReturnNode could be the exit of the FlowGraph, which
complicated some of the interval/structural analysis. Adding a single
common exit node helps reduce edge cases.

Also refactored:

Postdominators & loops can now be calculated on the FlowGraph at
construction. Previously it was "postdominator within this interval"
because there was not the common exit. This moved a lot of code out of
`if_statements.py` and into `flow_graph.py`.

`Node.emit_goto` is now only used for very special cases (certain asm
instructions, or the user manually annotates the source). Rather than
pre-compute which nodes need goto, we just use a goto if the node was
already emitted while building the Body.

The `emit_node()` function now contains the logic for checking if a node
has been already been emitted and a goto should be used instead.

Added `SwitchStatement` to `if_statements.py`.

Indenting is now done at formatting time, instead of being determined
while constructing `Body`s. This makes it possible to shift `Body`
segments around after construction without recalculating the indents.

Similiarly, empty return elision is done after building the `Body`s,
instead of during the `build_*` functions. Like indenting, this is to
accomidate re-writes.

- Add documentation/comments
- Remove stray debugging/comments
- Make `BaseNode` an ABC, `children()` abstract & return a list

- Make `indent` arg to `Formatter.indent()` optional
- Add more documentation, TODOs, and ext links

- Change locals from n/p/c to node/parent/child
- Emit comment about duplicated return
- Emit node content when skipped