Use symphonia::io::MediaSource for Reader extensions (#61)

This PR does the following:

 * Changes both `Reader::Extension` and `Reader::ExtensionSeek`  to use `symphonia::io::MediaSource`.
 * Removes the `File` and `Vec` variants of readers, instead opting to provide a `from_file` and `from_memory` associated function to create readers from the `File` and `Cursor<Vec<u8>>` implementations of `MediaSource`. 
 * Removes the ReadSeek trait.
 * Added a dependency on `symphonia_core`. This crate has no additional dependencies.

Cargo.toml

@@ -17,6 +17,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tracing = { version = "0.1", features = ["log"] }
 tracing-futures = "0.2"
+symphonia-core = "0.2"
 
 [dependencies.async-trait]
 optional = true