PiperOrigin-RevId: 615151423

PiperOrigin-RevId: 615198171

Still iterating on the design, but the basic idea is
- `pretty_printer.PrettyPrinter` contains functions to print types and variables
- Other classes in `pretty_printer` contain code to format the contents of
  structured error objects returned from various modules, e.g.
  `matcher.MatchError` or `function.FailedFunctionCall`. These classes take a
  `PrettyPrinter` as an init argument, and delegate to it to print types.
- The `ErrorLog` is initialised with a `PrettyPrinter`, which it passes to
  other `pretty_printer.*` objects it creates to format various errors.

Ultimately, all code that cares about the inner details of abstract objects
should be confined to the `PrettyPrinter` class, so that it can be swapped out
for a printer that works with the rewrite's abstract objects while keeping the
rest of the error formatting code unchanged.

PiperOrigin-RevId: 615473461

PiperOrigin-RevId: 615502466

PiperOrigin-RevId: 615542228

Also separates error printing into its own module to resolve an import cycle

PiperOrigin-RevId: 615548836

* Splits abstract.Instance into FrozenInstance and MutableInstance, so that we
  can easily distinguish between canonical instances created by
  BaseClass.instantiate() that should ignore any attribute setting done outside
  of __new__ and __init__, and instances created in the course of bytecode
  analysis that should respect all modifications.
* Implements opcodes for loading and storing attributes.

PiperOrigin-RevId: 615684582