pool: create a more efficient pool implementation

The current pool package only has a very basic implementation of pool:

```go
type Pool interface {
FreeAll()
Alloc(size int) []byte
Size() int
}
```

We want to minimise the amount of actual allocations, by saving the buffers we have already allocated to reuse them for future allocations required by parsers.

Use cases include:
* converting float64 to bytes
* converting int64 to bytes
* unquoting a string

Note: We do not want any concurrency, so we do not want to use sync.Pool.

See pool/basic.go for an example implementation that we a more efficient version of.

Start by reading the pool interface and the example implementation in pool/basic.go. Trace the listed parser use cases—float64 and int64 conversion and string unquoting—to understand the allocation patterns the pool must support. Done means providing a more efficient reusable pool implementation that minimizes allocations without introducing concurrency or sync.Pool.

Written by the indexing model from the issue text.