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.