protoc-gen-grpc-swift: FileHandle for stdin, stdout (grpc#1361)

This fixes a case where the output response is silently truncated.
Truncation occurs because the `write` syscall is only invoked a single
time, but `write` may return a value indicating a partial write for
various reasons, even in a blocking call. For example, if stdout is a
nonblocking pipe, output is truncated when the pipe is full.

Instead of rewriting Stdout.write to handle this case, let's migrate to
FileHandle for I/O, available as of Swift 3.2.

swift-protobuf has already migrated, and this commit is mostly a port of
github.com/apple/swift-protobuf/commit/2ccad3484e06d180af3bd7be53c96d41e8f03a62

Author: lrewega

Sources/protoc-gen-grpc-swift/io.swift

@@ -110,7 +110,7 @@ func main() throws {
   )
 
   // read plugin input
-  let rawRequest = try Stdin.readall()
+  let rawRequest = FileHandle.standardInput.readDataToEndOfFile()
   let request = try Google_Protobuf_Compiler_CodeGeneratorRequest(serializedData: rawRequest)
 
   let options = try GeneratorOptions(parameter: request.parameter)
@@ -137,7 +137,7 @@ func main() throws {
 
   // return everything to the caller
   let serializedResponse = try response.serializedData()
-  Stdout.write(bytes: serializedResponse)
+  FileHandle.standardOutput.write(serializedResponse)
 }
 
 do {