BREAKING CHANGE: Change contracts syntax to align with P2661R1

From `[[kind group: expression]]`

to `kind<group>(expression)`

Sorry for the breaking change to anyone who has been writing contacts. I'm trying to support gaining usage experience with P2961

Reference link: https://wg21.link/p2961r1

Author: hsutter

regression-tests/mixed-bounds-safety-with-assert-2.cpp2

@@ -9,8 +9,8 @@ main: () -> int = {
 
 add_42_to_subrange: (inout rng:_, start:int, end:int)
 = {
-    [[assert Bounds: 0 <= start]]
-    [[assert Bounds: end <= rng.ssize()]]
+    assert<Bounds>( 0 <= start )
+    assert<Bounds>( end <= rng.ssize() )
 
     count := 0;
     for  rng

regression-tests/mixed-bounds-safety-with-assert.cpp2

@@ -7,8 +7,8 @@ main: () -> int = {
 }
 
 print_subrange: (rng:_, start:int, end:int) = {
-    [[assert Bounds: 0 <= start]]
-    [[assert Bounds: end <= rng.ssize()]]
+    assert<Bounds>( 0 <= start )
+    assert<Bounds>( end <= rng.ssize() )
 
     count := 0;
     for  rng

regression-tests/mixed-bugfix-for-literal-as-nttp.cpp2

@@ -1,7 +1,7 @@
 #include <chrono>
 using namespace std::chrono_literals;
 main: () = {
-  [[assert: 10 as i32 == 10]]
-  [[assert: 10LL as i32 == 10]]
-  [[assert: 10s as std::chrono::seconds == 10s]]
+  assert( 10 as i32 == 10 )
+  assert( 10LL as i32 == 10 )
+  assert( 10s as std::chrono::seconds == 10s )
 }

regression-tests/mixed-captures-in-expressions-and-postconditions.cpp2

@@ -17,8 +17,8 @@ main: () -> int = {
 vec: std::vector<int> = ();
 
 insert_at: (where: int, val: int)
-    [[pre:  0 <= where && where <= vec.ssize()]]
-    [[post: vec.ssize() == vec.ssize()$ + 1]]
+    pre( 0 <= where && where <= vec.ssize() )
+    post( vec.ssize() == vec.ssize()$ + 1 )
 = {
     _ = vec.insert( vec.begin()+where, val );
 }

regression-tests/mixed-initialization-safety-1-error.cpp2

@@ -10,8 +10,8 @@ fill: (
     out x:     std::string,
     in  value: std::string,
     in  count: int
-    ) 
-    [[pre: value.size() >= count, "fill: value must contain at least count elements"]]
+    )
+    pre( value.size() >= count, "fill: value must contain at least count elements" )
 = {
     x = value.substr(0, count);
 }

regression-tests/mixed-initialization-safety-2-error.cpp2

@@ -10,8 +10,8 @@ fill: (
     out x:     std::string,
     in  value: std::string,
     in  count: int
-    ) 
-    [[pre: value.size() >= count, "fill: value must contain at least count elements"]]
+    )
+    pre( value.size() >= count, "fill: value must contain at least count elements" )
 = {
     x = value.substr(0, count);
 }

regression-tests/mixed-initialization-safety-3-contract-violation.cpp2

@@ -20,7 +20,7 @@ fill: (
     value: std::string,
     count: int
     )
-    [[pre: value.ssize() >= count, "fill: value must contain at least count elements"]]
+    pre( value.ssize() >= count, "fill: value must contain at least count elements" )
 = {
     x = value.substr(0, count);
 }

regression-tests/mixed-initialization-safety-3.cpp2

@@ -18,7 +18,7 @@ fill: (
     value: std::string,
     count: int
     )
-    [[pre: value.ssize() >= count, "fill: value must contain at least count elements"]]
+    pre( value.ssize() >= count, "fill: value must contain at least count elements" )
 = {
     x = value.substr(0, count);
 }

regression-tests/mixed-postexpression-with-capture.cpp2

@@ -11,9 +11,9 @@ main: () -> int = {
 
 vec: std::vector<int> = ();
 
-insert_at: (where: int, val: int) 
-    [[pre:  0 <= where && where <= vec.ssize()]]
-    [[post: vec.size() == vec.size()$ + 1]]
+insert_at: (where: int, val: int)
+    pre ( 0 <= where && where <= vec.ssize() )
+    post( vec.size() == vec.size()$ + 1 )
 = {
     vec.push_back(val);
 }

regression-tests/pure2-bugfix-for-assign-expression-list.cpp2

@@ -2,9 +2,9 @@ main: () = {
   vec: type == std::vector<int>;
   v: vec              = (0);
   v                   = ();
-  [[assert: v == :vec = ()]]
+  assert( v == :vec = () )
   v                   = (1);
-  [[assert: v == :vec = (1)]]
+  assert( v == :vec = (1) )
   v                   = (2, 3);
-  [[assert: v == :vec = (2, 3)]]
+  assert( v == :vec = (2, 3) )
 }