Add chr built-in function

```
chr(i)
```

This behaves similarly to Python's built-in `chr` function: for a given
integer `i`, it returns a string containing the character whose Unicode
code point is `i`. It is an error to provide an `i` that is not an
integer or falls outside the Unicode code point range.

docs/lexicon.html

@@ -47,6 +47,16 @@ <h2 class="title-2" id="python-builtins">
         - returns the length of <code class="code">x</code>.
       </span>
     </li>
+    <li>
+      <span>
+        <code class="code"
+          ><span class="fn-name">chr</span><span class="fn-p">(</span
+          ><span class="fn-arg">i</span><span class="fn-p">)</span></code
+        >
+        - returns a string containing the character whose Unicode code point
+        is the integer <code class="code">i</code>.
+      </span>
+    </li>
     <li>
       <span>
         <code class="code"

rules/builtins.build_defs

@@ -16,6 +16,8 @@ def build_rule(name:str, cmd:str|dict='', test_cmd:str|dict='', debug_cmd:str=''
                _subrepo:bool=False):
     pass
 
+def chr(i:int) -> str:
+    pass
 def len(obj:list|dict|str) -> int:
     pass
 def enumerate(seq:list):

src/parse/asp/builtins.go

@@ -11,6 +11,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"unicode"
 
 	"github.com/Masterminds/semver/v3"
 	"github.com/manifoldco/promptui"
@@ -47,6 +48,7 @@ func registerBuiltins(s *scope) {
 	setNativeCode(s, "all", allFunc)
 	setNativeCode(s, "min", min)
 	setNativeCode(s, "max", max)
+	setNativeCode(s, "chr", chr)
 	setNativeCode(s, "len", lenFunc)
 	setNativeCode(s, "glob", glob)
 	setNativeCode(s, "bool", boolType)
@@ -402,6 +404,13 @@ func objLen(obj pyObject) pyInt {
 	panic("object of type " + obj.Type() + " has no len()")
 }
 
+func chr(s *scope, args []pyObject) pyObject {
+	i, isInt := args[0].(pyInt)
+	s.Assert(isInt, "Argument i must be an integer, not %s", args[0].Type())
+	s.Assert(i >= 0 && i <= unicode.MaxRune, "Argument i must be within the Unicode code point range")
+	return pyString(i)
+}
+
 func isinstance(s *scope, args []pyObject) pyObject {
 	obj := args[0]
 	typesArg := args[1]

src/parse/asp/interpreter_test.go

@@ -487,6 +487,29 @@ func TestMax(t *testing.T) {
 	})
 }
 
+func TestChr(t *testing.T) {
+	t.Run("OK", func(t *testing.T) {
+		s, err := parseFile("src/parse/asp/test_data/interpreter/chr.build")
+		assert.NoError(t, err)
+		assert.EqualValues(t, pyString("\x00"), s.Lookup("null"))
+		assert.EqualValues(t, pyString("a"), s.Lookup("a"))
+		assert.EqualValues(t, pyString("€"), s.Lookup("euro"))
+		assert.EqualValues(t, pyString("\U0010FFFF"), s.Lookup("maximum"))
+	})
+	t.Run("Wrong parameter type", func(t *testing.T) {
+		_, err := parseFile("src/parse/asp/test_data/interpreter/chr_wrong_type.build")
+		assert.ErrorContains(t, err, "Invalid type for argument i to chr; expected int, was str")
+	})
+	t.Run("Parameter out of bounds (too low)", func(t *testing.T) {
+		_, err := parseFile("src/parse/asp/test_data/interpreter/chr_bounds_low.build")
+		assert.ErrorContains(t, err, "Argument i must be within the Unicode code point range")
+	})
+	t.Run("Parameter out of bounds (too high)", func(t *testing.T) {
+		_, err := parseFile("src/parse/asp/test_data/interpreter/chr_bounds_high.build")
+		assert.ErrorContains(t, err, "Argument i must be within the Unicode code point range")
+	})
+}
+
 func TestIsSemver(t *testing.T) {
 	t.Run("OK", func(t *testing.T) {
 		s, err := parseFile("src/parse/asp/test_data/interpreter/is_semver.build")

src/parse/asp/test_data/interpreter/chr.build

@@ -0,0 +1,4 @@
+null = chr(0)
+a = chr(97)
+euro = chr(8364)
+maximum = chr(1114111)

src/parse/asp/test_data/interpreter/chr_bounds_high.build

@@ -0,0 +1 @@
+x = chr(1114112)

src/parse/asp/test_data/interpreter/chr_bounds_low.build

@@ -0,0 +1 @@
+x = chr(-1)

src/parse/asp/test_data/interpreter/chr_wrong_type.build

@@ -0,0 +1 @@
+x = chr("x")

third_party/go/BUILD

@@ -83,7 +83,7 @@ go_repo(
 
 go_repo(
     module = "github.com/stretchr/testify",
-    version = "v1.7.0",
+    version = "v1.8.4",
 )
 
 go_repo(