idsig: make idsig file from apk

This CL adds the `create` and `write_into` methods to `V4Signature` each
of which is used to construct V4Signature from an apk and to write it
into a file.

Next step will be modifying the virtualization service to create an
idsig file for a given apk, thus eliminating the need to provide the
idsig file separately.

Bug: 193504400
Test: m libidsig libidsig.test

Change-Id: I2be60fbb6ec40af12297e20b112318a032dd78f9
diff --git a/idsig/Cargo.toml b/idsig/Cargo.toml
index 91b2842..49ae18d 100644
--- a/idsig/Cargo.toml
+++ b/idsig/Cargo.toml
@@ -6,6 +6,7 @@
 
 [dependencies]
 anyhow = "1.0"
+byteorder = "1.1"
 ring = "0.16"
 num-derive = "0.3"
 num-traits = "0.2"