Support more ID types, allow creating records with known IDs.

* Previously all ID types needed to have the `isZero` function defined.
  This was named after the UUID.isZero function but does not describe
  the general case. Non-numeric is types will not be "zero," they will
  be "empty." Renamed to `isEmpty` and implemented basic versions of
  this for `int`, `string`, and `UUID`.

* Previously newly created records were not allowed to have ids set.
  Fiber required them to be set in the database and the caller to
  retrieve the newly generated ID from the newly created record. This is
  not really a decision that Fiber should make in a general case. There
  are valid reasons for both possibilities (creating the id in the
  application code vs. creating the id in the database layer). As a
  more general-purpose solution Fiber now leaves this to the caller.

  If the caller provides an id value, Fiber will include it in the
  INSERT statement. If the id is unset, Fiber will not include it at
  all, allowing it to be generated in the database.

src/fiber_orm/util.nim

@@ -269,11 +269,13 @@ proc typeOfColumn*(modelType: NimNode, colName: string): NimNode =
       else: error "Unknown column type: " & $fieldType.getTypeInst
 
     else: return fieldType
-      
+
   raise newException(Exception,
     "model of type '" & $modelType & "' has no column named '" & colName & "'")
 
-proc isZero(val: int): bool = return val == 0
+proc isEmpty(val: int): bool = return val == 0
+proc isEmpty(val: UUID): bool = return val.isZero
+proc isEmpty(val: string): bool = return val.isNilOrWhitespace
 
 macro populateMutateClauses*(t: typed, newRecord: bool, mc: var MutateClauses): untyped =
 
@@ -285,14 +287,14 @@ macro populateMutateClauses*(t: typed, newRecord: bool, mc: var MutateClauses):
       # grab the field, it's string name, and it's type
       let fieldName = $fieldIdent
 
-      # we do not update the ID, but we do check: if we're creating a new
-      # record, we should not have an existing ID
+      # We only add clauses for the ID field if we're creating a new record and
+      # the caller provided a value..
       if fieldName == "id":
         result.add quote do:
-          if `newRecord` and not `t`.id.isZero:
-            raise newException(
-              AssertionError,
-              "Trying to create a new record, but the record already has an ID (" & $(`t`.id) & ").")
+          if `newRecord` and not `t`.id.isEmpty:
+            `mc`.columns.add(identNameToDb(`fieldName`))
+            `mc`.placeholders.add("?")
+            `mc`.values.add(dbFormat(`t`.`fieldIdent`))
 
       # if we're looking at an optional field, add logic to check for presence
       elif fieldType.kind == nnkBracketExpr and