Rewrite readFile and writeFile

[bartlomieju]

Closes #1989

[bartlomieju]

@ry it looks like open and writeFile have incompatible APIs...

For example:

const options: WriteFileOptions = {
   create: false,
   append: true,
}  // there is no `OpenMode` that would suffice this condition

Deno.writeFile("foo.txt", ..., options);

Should I handle that, by doing stat beforehand to check if file exists and bail out if it doesn't or would we rather change APIs?

EDIT: Please check writeFile in diff

[bartlomieju]

@ry ready for review

[bartlomieju]

@ry @kevinkassimo ready for round two

[ry]

Thank you - LGTM