Rename "local variables" to "explicit stack slots".

The term "local variables" predated the SSA builder in the front-end
crate, which also provides a way to implement source-language local
variables. The name "explicit stack slot" makes it clear what this
construct is.

cranelift/docs/example.cton

@@ -1,7 +1,7 @@
 test verifier
 
 function %average(i32, i32) -> f32 native {
-    ss1 = local 8            ; Stack slot for ``sum``.
+    ss1 = explicit_slot 8         ; Stack slot for ``sum``.
 
 ebb1(v1: i32, v2: i32):
     v3 = f64const 0x0.0