ripgrep: replace decoder with encoding_rs_io

This commit mostly moves the transcoder implementation to its own
crate: https://github.com/BurntSushi/encoding_rs_io

The new crate adds clear documentation and cleans up the implementation
to fully implement the contract of io::Read.

Cargo.toml

@@ -38,6 +38,7 @@ members = ["grep", "globset", "ignore"]
 atty = "0.2.10"
 bytecount = "0.3.1"
 encoding_rs = "0.8"
+encoding_rs_io = "0.1"
 globset = { version = "0.4.0", path = "globset" }
 grep = { version = "0.1.8", path = "grep" }
 ignore = { version = "0.4.0", path = "ignore" }