Introduce a #[declare_sql_function] attribute macro

This commit introduces a new `#[declare_sql_function]` attribute macro
that can be applied to `extern "SQL"` blocks. This is essentially the
same as the existing `define_sql_function!` function like macro in terms
of functionality.

I see the following advantages of using an attribute macro + an `extern
"SQL"` block instead:

* This is closer to rust syntax, so rustfmt will understand that and
work correctly inside these blocks
* This allows to put several functions into the same block
* Maybe in the future this also allows to apply attributes to the whole
block instead of to each item

The downside of this change is that we then have three variants to
declare sql functions:

* `sql_function!()` (deprectated)
* `define_sql_function!()` (introduced in 2.2, we might want to
deprecate that as well?)
* The new attribute macro

diesel/src/expression/functions/aggregate_folding.rs

@@ -1,7 +1,8 @@
-use crate::expression::functions::define_sql_function;
+use crate::expression::functions::declare_sql_function;
 use crate::sql_types::Foldable;
 
-define_sql_function! {
+#[declare_sql_function]
+extern "SQL" {
     /// Represents a SQL `SUM` function. This function can only take types which are
     /// Foldable.
     ///
@@ -19,9 +20,7 @@ define_sql_function! {
     /// ```
     #[aggregate]
     fn sum<ST: Foldable>(expr: ST) -> ST::Sum;
-}
 
-define_sql_function! {
     /// Represents a SQL `AVG` function. This function can only take types which are
     /// Foldable.
     ///

diesel/src/expression/functions/mod.rs

@@ -1,5 +1,7 @@
 //! Helper macros to define custom sql functions
 
+#[doc(inline)]
+pub use diesel_derives::declare_sql_function;
 #[doc(inline)]
 pub use diesel_derives::define_sql_function;