Switch to 2021 edition

[GuillaumeGomez]

Code updates were done with this python script:

import os

def replace_file_content(path):
    if not path.endswith('.rs'):
        return
    with open(path, 'r') as f:
        content = f.read()
    x = (content.replace('::std::', 'std::')
            .replace('&::', '&crate::')
            .replace(' ::', ' crate::')
            .replace('(::', '(crate::')
            .replace('crate::sys', 'sys')
            .replace('crate::paste', 'paste')
            .replace('use types', 'use crate::types')
            .replace('use ffi', 'use crate::ffi')
            .replace('use m', 'use crate::m')
            .replace('use r', 'use crate::r')
            .replace('use C', 'use crate::C')
            .replace('use {', 'use crate::{'))
    if x == content:
        return
    with open(path, 'w') as f:
        f.write(x)

def recurse(path):
    entries = os.listdir(path)
    for d in entries:
        d = os.path.join(path, d)
        if os.path.isfile(d):
            replace_file_content(d)
        else:
            recurse(d)

recurse('src')