extracted NewRSRC() function
binutil: factored out from rsrc

--HG--
rename : rsrc.go => binutil/plain.go
rename : walk.go => binutil/walk.go
binutil: moved Writer and SizedOpen from rsrc

--HG--
rename : sizedfile.go => binutil/sizedfile.go
rename : writer.go => binutil/writer.go
coff: extracted a library

--HG--
rename : rsrc.go => coff/coff.go
coff: explicitly use interface Sizer
updated README & USAGE