Some minor edits here to make things suitable for Base.

Use handle_message rather than dispatch_message, as not all message
handling is pure dispatch (the message may be stored rather than sent
elsewhere).

Reuse the name dispatch_message internally.

* Forward depwarn() calls to new backend
* Use caller frame pointer as message id.  This makes initial
  first-pass deduplication of depwarns quite a lot faster.
* Change --depwarn=error to --depwarn=throw for clarity
* Introduce defines for log levels into julia.h

Forward all frontend depwarn messages through to the julia logging
system for consistency of formatting and dispatch.  Deciding whether a
depwarn should appear as an thrown error is also now handled outside
flisp to keep this decision in one place where possible.

Detail:

* Ensure that deprecation-message forwards to the logging system.  Also
  include file and line number metadata
* Remove jl_parse_depwarn(), flisp *depwarn* and related scheme code as
  this can be handled on the julia side.
* Rename syntax_deprecation_warnings(false) to
  without_syntax_deprecations, as this was only ever used to turn syntax
  deprecations off.
* Ensure that all lowering depwarns with existing line number
  information get this passed through as accessible metadata rather than
  as a string.
* Use distinct functions for depwarns coming from the parser vs
  lowering, as these get line number information in a different way.

TODO:
* Figure out a decent `id` for the syntax and lowering depwarn messages
* Make without_syntax_deprecations filter only depwarn messages, rather
  than everything!

* Additional exports
* Fallback error message formatting
* Minor global logger init min level bug

For now this seems like the right thing as error and informational
reporting is a core requirement for a compiler - parts of the frontend
and runtime now call the logging system when it's available.

These can be used to transform a lot of printf()'s into log statements.

This allows users to control whether a custom logger type will catch
exceptions during message creation and log them, or simply allow them to
pass through.

Provides a general tool to log information from the flisp frontend code.
Make the syntax deprecations use this instead of using the purpose built
syntax_depwarn function on the julia side.

Also reinstate flisp-level awareness of the depwarn level.  It's just
simpler.

This is purely for implementation convenience.  It restricts the levels
artifically, but it's hard to imagine what anyone would do with more than
a million custom levels.

This can be handled in a much nicer way by installing a logger in-process.

TODO: Define such a logger and test it!

Port from MicroLogging refactoring

Also rearrange exports to avoid exporting AbstractLogger-related methods.