The type of scanning buffers. A scanning buffer is the argument passed
to the scanning functions used by the scanf family of functions.
The scanning buffer holds the current state of the scan, plus
a function to get the next char from the input, and a token buffer
to store the string matched so far.
Scanning.from_string s returns a scanning buffer which reads
from the given string.
Reading starts from the first character in the string.
The end-of-input condition is set when the end of the string is reached.
Scanning.from_function f returns a scanning buffer with
the given function as its reading method.
When scanning needs one more character, the given function is called.
When the function has no more character to provide, it must set
an end-of-input condition by raising the exception End_of_file.