[FunC] Deprecate method_id specifier, introduce get keyword

`get` keyword behaves exactly like `method_id` (auto-calc hash),
but it's placed on the left, similar to Tact: `get T name()`.

`method_id(n)` is still valid, considering it can't be invoked by name,
since a client will compute another hash.
It's supposed it will be still used in tests and in low-level code
(not to be called externally, but to be called after replacing c3).

`get(hash)` is invalid, this keyword does not accept anything.

crypto/func/auto-tests/tests/camel4.fc

@@ -40,7 +40,7 @@ int test1() method_id(101) {
     return s~load_uint(32);
 }
 
-int test2(int ret) method_id {
+get int test2(int ret) {
     return setAndGetDataWrapper(ret);
 }
 

crypto/func/auto-tests/tests/invalid-get-method-1.fc

@@ -0,0 +1,8 @@
+get (int, int) hello(int x, int y) method_id(123) {
+    return (x, y);
+}
+
+{-
+@compilation_should_fail
+@stderr both `get` and `method_id` are not allowed
+-}

crypto/func/auto-tests/tests/s1.fc

@@ -1,28 +1,28 @@
-slice ascii_slice() method_id {
+get slice ascii_slice() {
   return "string";
 }
 
-slice raw_slice() method_id {
+get slice raw_slice() {
   return "abcdef"s;
 }
 
-slice addr_slice() method_id {
+get slice addr_slice() {
   return "Ef8zMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzMzM0vF"a;
 }
 
-int string_hex() method_id {
+get int string_hex() {
   return "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345"u;
 }
 
-int string_minihash() method_id {
+get int string_minihash() {
   return "transfer(slice, int)"h;
 }
 
-int string_maxihash() method_id {
+get int string_maxihash() {
   return "transfer(slice, int)"H;
 }
 
-int string_crc32() method_id {
+get int string_crc32() {
   return "transfer(slice, int)"c;
 }