Optimize let expressions as frame slot vars (#1622)

This introduces an optimization to write let expressions to frame slots
of the current frame, rather than transforming them into lambdas. As a
result, let expressions are _much_ faster to execute.

pkl-core/src/test/files/LanguageSnippetTests/input/basic/let.pkl

@@ -10,24 +10,25 @@ res2 =
 
 res3 =
   let (x = 1)
-    let (y = 2)
-      x + y + x
+  let (y = 2)
+    x + y + x
 
 res4 =
   let (x = 1)
-    let (x = 2)
-      x + x
+  let (x = 2)
+    x + x
 
 res5 =
-  let (price = 500) new {
-    lowestPrice = price - 100
-    averagePrice = new {
-      price
+  let (price = 500)
+    new {
+      lowestPrice = price - 100
+      averagePrice = new {
+        price
+      }
+      highestPrice = new {
+        ["price"] = price + 100
+      }
     }
-    highestPrice = new {
-      ["price"] = price + 100
-    }
-  }
 
 res6 =
   let (str = "Pigeon".reverse())
@@ -51,8 +52,9 @@ local g = (a) ->
 
 res9 = g.apply(3)
 
-local h = let (b = 2)
-  (a) -> a + b
+local h =
+  let (b = 2)
+    (a) -> a + b
 
 res10 = h.apply(3)
 
@@ -71,9 +73,9 @@ res12 = new Lets {
 
 res13 =
   let (x = 1)
-    let (y = x)
-      let (z = y)
-        x + y + z
+  let (y = x)
+  let (z = y)
+    x + y + z
 
 // x can't access y
 res14 = test.catch(() -> let (x = y) let (y = 2) x + y)

pkl-core/src/test/files/LanguageSnippetTests/input/errors/letExpressionError1.pkl

@@ -1,4 +1,4 @@
 res1 =
   let (x = 1)
-    let (y = 2)
-      throw("ouch")
+  let (y = 2)
+    throw("ouch")

pkl-core/src/test/files/LanguageSnippetTests/output/errors/letExpressionError1.err

@@ -3,15 +3,11 @@ ouch
 
 x | throw("ouch")
     ^^^^^^^^^^^^^
-at letExpressionError1#res1.<function#2> (file:///$snippetsDir/input/errors/letExpressionError1.pkl)
-
-x | let (y = 2)
-    ^^^^^^^^^^^
-at letExpressionError1#res1.<function#1> (file:///$snippetsDir/input/errors/letExpressionError1.pkl)
+at letExpressionError1#res1 (file:///$snippetsDir/input/errors/letExpressionError1.pkl)
 
 x | let (x = 1)
     ^^^^^^^^^^^
-at letExpressionError1#res1 (file:///$snippetsDir/input/errors/letExpressionError1.pkl)
+at letExpressionError1#res1.<let expr> (file:///$snippetsDir/input/errors/letExpressionError1.pkl)
 
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

pkl-core/src/test/files/LanguageSnippetTests/output/errors/letExpressionError2.err

@@ -3,11 +3,11 @@ ouch
 
 x | let (y = throw("ouch"))
              ^^^^^^^^^^^^^
-at letExpressionError2#res1.<function#1> (file:///$snippetsDir/input/errors/letExpressionError2.pkl)
+at letExpressionError2#res1 (file:///$snippetsDir/input/errors/letExpressionError2.pkl)
 
 x | let (x = 1)
     ^^^^^^^^^^^
-at letExpressionError2#res1 (file:///$snippetsDir/input/errors/letExpressionError2.pkl)
+at letExpressionError2#res1.<let expr> (file:///$snippetsDir/input/errors/letExpressionError2.pkl)
 
 xxx | renderer.renderDocument(value)
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

pkl-core/src/test/files/LanguageSnippetTests/output/errors/letExpressionErrorTyped.err

@@ -4,10 +4,6 @@ Value: "abc"
 
 x | let (x: Float = "abc")
             ^^^^^
-at letExpressionErrorTyped#res1.<function#1> (file:///$snippetsDir/input/errors/letExpressionErrorTyped.pkl)
-
-x | let (x: Float = "abc")
-    ^^^^^^^^^^^^^^^^^^^^^^
 at letExpressionErrorTyped#res1 (file:///$snippetsDir/input/errors/letExpressionErrorTyped.pkl)
 
 xxx | renderer.renderDocument(value)