rustdoc-search: add slices and arrays to index

This indexes them as primitives with generics, so `slice<u32>` is
how you search for `[u32]`, and `array<u32>` for `[u32; 1]`.
A future commit will desugar the square bracket syntax to search
both arrays and slices at once.

tests/rustdoc-js/slice-array.rs

@@ -0,0 +1,16 @@
+pub struct P;
+pub struct Q;
+pub struct R<T>(T);
+
+// returns test
+pub fn alef() -> &'static [R<P>] { loop {} }
+pub fn bet() -> R<[Q; 32]> { loop {} }
+
+// in_args test
+pub fn alpha(_x: R<&'static [P]>) { loop {} }
+pub fn beta(_x: [R<Q>; 32]) { loop {} }
+
+pub trait TraitCat {}
+pub trait TraitDog {}
+
+pub fn gamma<T: TraitCat + TraitDog>(t: [T; 32]) {}