Feature Request: LEB128 integers

It would be great if there was a way to parse LEB128 integers. These are very common in formats like Parquet (which I'm building a parser for), and Protobuf.

```
extension UInt64 {

public init(parsing input: inout ParserSpan) throws {

var value: UInt64 = 0
var shift: UInt64 = 0

while true {
guard shift <= 63 else { // 63 is the max shift for a UInt64
// The varint is too long to fit in a UInt64.
throw BinaryParsing.error("Varint too long to fit in UInt64")
}

let byte = try UInt8(parsingLittleEndian: &input, byteCount: 1)

value |= UInt64(byte & 0x7f) << shift

if (byte & 0x80) == 0 {
self = value
return
}

shift += 7
}
}
}
```

Start by reviewing the existing UInt64 parsing APIs in the swift-binary-parsing repository and the proposed UInt64 init(parsing:) entry point shown in the issue. Check how parsing errors and integer parsing tests are organized; done means LEB128 values from formats such as Parquet and Protobuf can be parsed, with oversized values rejected as described.

Written by the indexing model from the issue text.