refactor!: rework app router relationship

[adriangb]

This change:

• Moves the responsibility of middleware to Router, effectively enabling Router-level middleware that will work with Mount, Host, etc.
• Moves the responsibility of lifespans into App since there can only ever be 1 lifespan and there can only be 1 App.

As part of this, App no longer inherits from Starlette and Router no longer inherits from its counterpart in Starlette.
This both simplifies things and complicates them a bit, but it does give us the opportunity to do a bit of cleanup and performance optimization (slots, removing lifespan wrappers and making routing sans-io)

There are obviously cons to this though. I might let this stew a bit and see.

Here are the benchmark numbers using the benchmarks in /benchmark:

	/simple	/fast_deps	/slow_deps
main	2.19	1.44	0.16
this	2.40	1.38	0.16
starlette	4.47
fastapi	2.76	0.63	.08

So it looks like this is a pretty solid +10% performance boost for simple scenarios

[codecov-commenter]

Codecov Report

Merging #32 (378df0f) into main (7985030) will decrease coverage by 0.49%.
The diff coverage is 73.21%.

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   98.11%   97.62%   -0.50%     
==========================================
  Files         172      172              
  Lines        5412     5473      +61     
  Branches      624      639      +15     
==========================================
+ Hits         5310     5343      +33     
- Misses         56       78      +22     
- Partials       46       52       +6     

Impacted Files	Coverage Δ
xpresso/routing/router.py	66.12% <63.79%> (-33.88%)	⬇️
xpresso/applications.py	90.24% <80.43%> (-5.30%)	⬇️
xpresso/_utils/routing.py	96.66% <100.00%> (+0.11%)	⬆️
xpresso/exception_handlers.py	100.00% <100.00%> (ø)
xpresso/responses.py	100.00% <100.00%> (ø)