Add Generate() function for ergonomic stream creation

[destel]

Adds a new Generate function that simplifies stream creation by:

• Removing channel/goroutine boilerplate
• Providing ergonomic send/sendErr callbacks
• Managing channel lifecycle automatically

Before:

stream := make(chan rill.Try[int])
go func() {
	defer close(stream)
	for i := 0; i < 100; i++ {
		stream <- rill.Try[int]{Value: i}
	}
	stream <- rill.Try[int]{Error: someError}
}()

After:

stream := rill.Generate(func(send func(int), sendErr func(error)) {
	for i := 0; i < 100; i++ {
		send(i)
	}
	sendErr(someError)
})

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.11%. Comparing base (514ea1c) to head (a593797).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   99.10%   99.11%   +0.01%     
==========================================
  Files          17       17              
  Lines        1002     1015      +13     
==========================================
+ Hits          993     1006      +13     
  Misses          9        9              

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