Convert the code to use non-partial functions from Protolude.StringConv

[vu3rdd]

With Protolude 0.3.0, a non-partial toS function is exposed. This breaks the build (see #44). While we can fix #44 by using toS from Protolude.Conv (which needs to be explicitly exposed), there is a good chance that it may get removed from Protolude. So, we need to look into using the non-partial text/bytestring conversion.