Add support for multiple simultaneous database features by using macros.

Diesel requires the following changes:
- Separate connection and pool types per connection, the generate_connections! macro generates an enum with a variant per db type
- Separate migrations and schemas, these were always imported as one type depending on db feature, now they are all imported under different module names
- Separate model objects per connection, the db_object! macro generates one object for each connection with the diesel macros, a generic object, and methods to convert between the connection-specific and the generic ones
- Separate connection queries, the db_run! macro allows writing only one that gets compiled for all databases or multiple ones

build.rs

@@ -1,13 +1,14 @@
 use std::process::Command;
 use std::env;
 
-fn main() {
-    #[cfg(all(feature = "sqlite", feature = "mysql"))]
-    compile_error!("Can't enable both sqlite and mysql at the same time");
-    #[cfg(all(feature = "sqlite", feature = "postgresql"))]
-    compile_error!("Can't enable both sqlite and postgresql at the same time");
-    #[cfg(all(feature = "mysql", feature = "postgresql"))]
-    compile_error!("Can't enable both mysql and postgresql at the same time");
+fn main() { 
+    // This allow using #[cfg(sqlite)] instead of #[cfg(feature = "sqlite")], which helps when trying to add them through macros
+    #[cfg(feature = "sqlite")]
+    println!("cargo:rustc-cfg=sqlite");
+    #[cfg(feature = "mysql")]
+    println!("cargo:rustc-cfg=mysql");
+    #[cfg(feature = "postgresql")]
+    println!("cargo:rustc-cfg=postgresql");
 
     #[cfg(not(any(feature = "sqlite", feature = "mysql", feature = "postgresql")))]
     compile_error!("You need to enable one DB backend. To build with previous defaults do: cargo build --features sqlite");