Allow '_' in label and meta names

examples/stack.mimasm

@@ -163,11 +163,11 @@ callee:
   STV tmp3 ; For comparison in loop
   STV tmp4 ; For accumulating result
 
-  callee-loop:
+  callee_loop:
   ; Break if counter (tmp1) is zero
   LDV tmp1
   EQL tmp3
-  JMN callee-loop-end
+  JMN callee_loop_end
   ; Decrement counter (tmp1) by 1
   LDV tmp1
   ADC -1
@@ -177,9 +177,9 @@ callee:
   ADD tmp2
   STV tmp4
   ; And repeat the loop
-  JMP callee-loop
+  JMP callee_loop
   
-  callee-loop-end:
+  callee_loop_end:
   ; Save the result in return value 2
   LDV tmp4
   STRF 2

src/Mima/Asm/Phase1.hs

@@ -213,7 +213,7 @@ withSpan f = do
 name :: Parser (Name Span)
 name = fmap (uncurry Name) $ withSpan $ do
   firstChar <- satisfy isLower <?> "lowercase character"
-  otherChars <- takeWhileP (Just "alphanumeric character") isAlphaNum
+  otherChars <- takeWhileP (Just "alphanumeric character or '_'") (\c -> isAlphaNum c || c == '_')
   pure $ T.pack [firstChar] <> otherChars
 
 number :: (Num a) => Parser a