[BUG] Performance improvement for non-uvicorn ASGI environments

[ryansb]

Describe the bug

fasthtml/core.py file imports uvicorn even though it's only used for serve() calls. For AWS Lambda this causes worse cold starts than if the uvicorn was imported inline of serve().

Minimal Reproducible Example

# Your code here
from fasthtml.core import *
uvicorn # uvicorn is imported and re-exported, even though it's only used in `serve()`

Expected behavior

I'd expect that users of other ASGI serving methods would not need to load uvicorn at runtime unless they are using it.

Environment Information
Please provide the following version information:

• fastlite version: 0.1.1
• fastcore version: 1.7.28
• fasthtml version: 0.10.3

Confirmation
Please confirm the following:

• I have read the FAQ (https://docs.fastht.ml/explains/faq.html)
• I have provided a minimal reproducible example
• I have included the versions of fastlite, fastcore, and fasthtml
• I understand that this is a volunteer open source project with no commercial support.

Additional context
Add any other context about the problem here.

Screenshots
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