remove std.fs.Dir.readFileAllocOptions, replace with more flexible API
readFileIntoArrayList
remove std.fs.File.readToEndAllocOptions, replace with more flexible API
readIntoArrayList
update std.fs.File to new reader/writer API
add helper functions to std.io.Reader.Limit
replace std.io.Writer.FileLen with std.io.Reader.Limit
make offset a type rather than u64 so that it can distinguish between
streaming read and positional read
avoid an unnecessary allocation in std.zig.readSourceFileToEndAlloc when
there is a UTF-16 little endian BOM.
