Convert double to integer via truncation. (#192)

* Convert double to integer via truncation.

Matches behavior of C, Go, and Java.

Introduce a round() function for round-half-away like GoogleSQL.

* Address review feedback.

Defer round() until another PR.

Co-authored-by: Jim Larson <jimlarson@google.com>

doc/langdef.md

@@ -2070,7 +2070,8 @@ See [cel-go/issues/9](https://github.com/google/cel-go/issues/9).
       (double) -> int
     </td>
     <td>
-      type conversion
+      Type conversion. Rounds toward zero, then errors if result is out of
+      range.
     </td>
   </tr>
   <tr>
@@ -2295,7 +2296,8 @@ See [cel-go/issues/9](https://github.com/google/cel-go/issues/9).
       (double) -> uint
     </td>
     <td>
-      type conversion
+      Type conversion. Rounds toward zero, then errors if result is out of
+      range.
     </td>
   </tr>
   <tr>

tests/simple/testdata/conversions.textproto

@@ -174,24 +174,24 @@ section {
     value: { int64_value: -123 }
   }
   test {
-    name: "double_nearest"
+    name: "double_truncate"
     expr: "int(1.9)"
-    value: { int64_value: 2 }
+    value: { int64_value: 1 }
   }
   test {
-    name: "double_nearest_neg"
+    name: "double_truncate_neg"
     expr: "int(-7.9)"
-    value: { int64_value: -8 }
+    value: { int64_value: -7 }
   }
   test {
-    name: "double_half_away_pos"
+    name: "double_half_pos"
     expr: "int(11.5)"
-    value: { int64_value: 12 }
+    value: { int64_value: 11 }
   }
   test {
-    name: "double_half_away_neg"
+    name: "double_half_neg"
     expr: "int(-3.5)"
-    value: { int64_value: -4 }
+    value: { int64_value: -3 }
   }
   test {
     name: "double_big_exact"
@@ -447,19 +447,14 @@ section {
     value: { uint64_value: 3 }
   }
   test {
-    name: "double_nearest_int"
-    expr: "int(1.9)"
-    value: { int64_value: 2 }
-  }
-  test {
-    name: "double_nearest"
+    name: "double_truncate"
     expr: "uint(1.9)"
-    value: { uint64_value: 2 }
+    value: { uint64_value: 1 }
   }
   test {
-    name: "double_half_away"
+    name: "double_half"
     expr: "uint(25.5)"
-    value: { uint64_value: 26 }
+    value: { uint64_value: 25 }
   }
   test {
     name: "double_big_exact"
@@ -474,8 +469,8 @@ section {
     value { uint64_value: 36028797018963968 }
   }
   test {
-    name: "double_range_beyond_int"
-    description: "Checks conversion of integer outside int range."
+    name: "double_range_beyond_uint"
+    description: "Checks conversion of integer outside uint range."
     expr: "uint(1e19)"
     value: { uint64_value: 10000000000000000000 }
   }