io: add new stringreader struct

[Casper64]

This pr adds a new StringReader struct to the io module.

The StringRedeader is able to read data from a Reader interface and/or source string to a dynamically growing buffer using a string builder.

Unlike the BufferedReader, StringReader will keep the entire contents of the buffer in memory, allowing the incoming data to be reused and read in an efficient matter.

The StringReader also implements the io.ReaderWriter interface

[blackshirt]

If this reader keep the entire buffers content in memory, i think its a good to place limit on them, or it would grow uncontrollable

[JalonSolov]

Since it's a Reader, it's on the caller to control the size of the string. If it was a Writer, then yes, it would have to handle not allowing the string to run you out of memory.

[Casper64]

if you want to control how many bytes are read in total, you can call ‘fill_buffer_until`. This function will only read a maximum amount of bytes

[Casper64]

That said, it would be useful to provide methods to clear the buffer and offset. I'll add them

[Casper64]

I've added a line in the doc comment to make it more clear:

The StringReader will not set a maximum capacity to the string builders buffer and could grow very large.

[spytheman]

Excellent work.