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/filetests/parser/tiny.cton

@@ -124,7 +124,7 @@ ebb0(v90: i32, v91: f32):
 ; Stack slot references
 function %stack() {
     ss10 = spill_slot 8
-    ss2 = local 4
+    ss2 = explicit_slot 4
     ss3 = incoming_arg 4, offset 8
     ss4 = outgoing_arg 4
     ss5 = emergency_slot 4
@@ -136,7 +136,7 @@ ebb0:
     stack_store v2, ss2
 }
 ; sameln: function %stack() native {
-; check:     ss2 = local 4
+; check:     ss2 = explicit_slot 4
 ; check:     ss3 = incoming_arg 4, offset 8
 ; check:     ss4 = outgoing_arg 4
 ; check:     ss5 = emergency_slot 4