SpanBuilder

### Overview

A `StringBuilder`-like api, but for spans.

### API breakdown

```cs
public ref struct SpanBuilder
{
Span span;
int length;

public SpanBuilder(Span storage) { span = storage; }
public SpanBuilder(int capacity) { span = new T[capacity]; }

public void Add(in T @value)
{
EnsureCapacity(length + 1);
span[length++] = @value;
}
public EnsureCapacity(int capacity)
{
if(span.Length >= capacity) return;
Span newSpan = new T[Math.Pow(2, BitOperations.LeadingZeroCount(checked((uint)capacity)))];
span.Slice(0, length).CopyTo(newSpan);
span = newSpan;
}
public Span Build(){
Span built = span.Slice(0, length);
span = default;
length = default;
return built;
}
}
```

### Usage example

// too big 😅

### Breaking change?

No

### Alternatives

# `List`

- causes allocation by itself (isn't a value type)
- doesn't support use of existing storage

# StringBuilder

- only for strings

# ValueStringBuilder

- is internal

### Additional context

_No response_

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