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/isa/intel/prologue-epilogue.cton

@@ -4,13 +4,13 @@ set is_compressed
 isa intel haswell
 
 function %foo() {
-    ss0 = local 168
+    ss0 = explicit_slot 168
 ebb0:
     return
 }
 
 ; check: function %foo(i64 fp [%rbp], i64 csr [%rbx], i64 csr [%r12], i64 csr [%r13], i64 csr [%r14], i64 csr [%r15]) -> i64 fp [%rbp], i64 csr [%rbx], i64 csr [%r12], i64 csr [%r13], i64 csr [%r14], i64 csr [%r15] native {
-; nextln:     ss0 = local 168, offset -224
+; nextln:     ss0 = explicit_slot 168, offset -224
 ; nextln:     ss1 = incoming_arg 56, offset -56
 ; check: ebb0(v0: i64 [%rbp], v1: i64 [%rbx], v2: i64 [%r12], v3: i64 [%r13], v4: i64 [%r14], v5: i64 [%r15]):
 ; nextln:     x86_push v0
@@ -29,4 +29,4 @@ ebb0:
 ; nextln:     v7 = x86_pop.i64
 ; nextln:     v6 = x86_pop.i64
 ; nextln:     return v6, v7, v8, v9, v10, v11
-; nextln: }
+; nextln: }

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