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.
The compiler actually doesn't need any functional changes for this: Sema
does reification based on the tag indices of `std.builtin.Type` already!
So, no zig1.wasm update is necessary.
This change is necessary to disallow name clashes between fields and
decls on a type, which is a prerequisite of #9938.
