Merge 'Added 'likelihood' scalar function' from Sachin Kumar Singh

The `likelihood(X,Y)` function returns argument X unchanged. The value Y
in likelihood(X,Y) must be a floating point constant between 0.0 and
1.0, inclusive.
```
sqlite> explain SELECT likelihood(42, 0.0);
addr  opcode         p1    p2    p3    p4             p5  comment
----  -------------  ----  ----  ----  -------------  --  -------------
0     Init           0     6     0                    0   Start at 6
1     Once           0     3     0                    0
2     Integer        42    2     0                    0   r[2]=42
3     Copy           2     1     0                    0   r[1]=r[2]
4     ResultRow      1     1     0                    0   output=r[1]
5     Halt           0     0     0                    0
6     Goto           0     1     0                    0
```
```
limbo> explain SELECT likelihood(42, 0.0);
addr  opcode             p1    p2    p3    p4             p5  comment
----  -----------------  ----  ----  ----  -------------  --  -------
0     Init               0     4     0                    0   Start at 4
1     Copy               2     1     0                    0   r[1]=r[2]
2     ResultRow          1     1     0                    0   output=r[1]
3     Halt               0     0     0                    0
4     Integer            42    2     0                    0   r[2]=42
5     Goto               0     1     0                    0
```

Closes #1303

testing/scalar-functions.test

@@ -211,6 +211,38 @@ do_execsql_test likely-null {
     select likely(NULL)
 } {}
 
+do_execsql_test likelihood-string {
+  SELECT likelihood('limbo', 0.5);
+} {limbo}
+
+do_execsql_test likelihood-string-high-probability {
+  SELECT likelihood('database', 0.9375);
+} {database}
+
+do_execsql_test likelihood-integer {
+  SELECT likelihood(100, 0.0625);
+} {100}
+
+do_execsql_test likelihood-integer-probability-1 {
+  SELECT likelihood(42, 1.0);
+} {42}
+
+do_execsql_test likelihood-decimal {
+  SELECT likelihood(12.34, 0.5);
+} {12.34}
+
+do_execsql_test likelihood-null {
+  SELECT likelihood(NULL, 0.5);
+} {}
+
+do_execsql_test likelihood-blob {
+  SELECT hex(likelihood(x'01020304', 0.5));
+} {01020304}
+
+do_execsql_test likelihood-zero-probability {
+  SELECT likelihood(999, 0.0);
+} {999}
+
 do_execsql_test unhex-str-ab {
   SELECT unhex('6162');
 } {ab}