Treat hundred, thousand, … as units

examples/consistency_si.nbt

@@ -1,5 +1,3 @@
-assert_eq(1 rad , 1 m² / m²)
-assert_eq(1 sr  , 1 m² / m²)
 assert_eq(1 Hz  , 1 / s)
 assert_eq(1 N   , 1 kg * m / s^2)
 assert_eq(1 Pa  , 1 N / m²)

numbat/modules/math/constants.nbt

@@ -13,13 +13,13 @@ let golden_ratio = φ
 
 #### Large numbers
 
-let hundred =  100
-let thousand =  1_000
-let million =  1_000_000
-let billion =  10^9
-let trillion =  10^12
-let quadrillion =  10^15
-let quintillion =  10^18
+unit hundred =  100
+unit thousand =  1_000
+unit million =  1_000_000
+unit billion =  10^9
+unit trillion =  10^12
+unit quadrillion =  10^15
+unit quintillion =  10^18
 
 let googol =  10^100
 
@@ -59,5 +59,4 @@ let half = 1 / 2
 let semi = 1 / 2
 let double = 2
 let triple = 3
-let dozen = 12
-
+unit dozen = 12

numbat/tests/interpreter.rs

@@ -103,6 +103,7 @@ fn test_conversions() {
     expect_output("5m^2 -> m*cm", "500 m·cm");
     expect_output("5m^2 -> cm*m", "500 cm·m");
     expect_output("1 kB / 10 ms -> MB/s", "0.1 MB/s");
+    expect_output("55! / (6! (55 - 6)!) -> million", "28.9897 million");
 }
 
 #[test]