YamlRenderer: allow all primitive scalar types as map keys (#879)

pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlRenderer8.yml.pkl

@@ -0,0 +1,33 @@
+// Test that non-String scalars can be rendered as YAML map keys.
+mappingInt = new Mapping {
+  [5] = "abc"
+  [10] = "def"
+  [20] = "ghi"
+}
+
+mapInt = mappingInt.toMap()
+
+mappingFloat = new Mapping {
+  [5.0] = "abc"
+  [10.5] = "def"
+  [20.999999990] = "ghi"
+}
+
+mapFloat = mappingFloat.toMap()
+
+mappingBool = new Mapping {
+  [true] = "abc"
+  [false] = "def"
+}
+
+mapBool = mappingBool.toMap()
+
+mappingNull = new Mapping {
+  [null] = "abc"
+}
+
+mapNull = mappingNull.toMap()
+
+output {
+  renderer = new YamlRenderer {}
+}

pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlRenderer8.yml

@@ -0,0 +1,26 @@
+mappingInt:
+  5: abc
+  10: def
+  20: ghi
+mapInt:
+  5: abc
+  10: def
+  20: ghi
+mappingFloat:
+  5.0: abc
+  10.5: def
+  20.99999999: ghi
+mapFloat:
+  5.0: abc
+  10.5: def
+  20.99999999: ghi
+mappingBool:
+  true: abc
+  false: def
+mapBool:
+  true: abc
+  false: def
+mappingNull:
+  null: abc
+mapNull:
+  null: abc