fix(lambda): environment var values are strings (aws#3858)

* fix(lambda): environment var values are strings

AWS Lambda requires that all environment variables will be
strings but the API indicated `any` as the type of the value.
If users would pass in a non-string value, they would see an
error only during deployment.

Fixes aws#3337

* remove redundant null-check on this.environment

* add breaking change excludes

allowed-breaking-changes.txt

@@ -12,3 +12,6 @@ removed:@aws-cdk/aws-apigateway.HttpIntegration.props
 removed:@aws-cdk/aws-apigateway.Integration.props
 removed:@aws-cdk/aws-apigateway.LambdaIntegration.props
 removed:@aws-cdk/aws-apigateway.MockIntegration.props
+incompatible-argument:@aws-cdk/aws-lambda.Function.<initializer>
+incompatible-argument:@aws-cdk/aws-lambda.SingletonFunction.<initializer>
+incompatible-argument:@aws-cdk/aws-lambda.Function.addEnvironment

packages/@aws-cdk/aws-lambda/lib/function.ts

@@ -76,7 +76,7 @@ export interface FunctionProps {
    *
    * @default - No environment variables.
    */
-  readonly environment?: { [key: string]: any };
+  readonly environment?: { [key: string]: string };
 
   /**
    * The runtime environment for the Lambda function that you are uploading.
@@ -392,7 +392,7 @@ export class Function extends FunctionBase {
   /**
    * Environment variables for this function
    */
-  private readonly environment?: { [key: string]: any };
+  private readonly environment: { [key: string]: string };
 
   constructor(scope: Construct, id: string, props: FunctionProps) {
     super(scope, id, {
@@ -491,11 +491,7 @@ export class Function extends FunctionBase {
    * @param key The environment variable key.
    * @param value The environment variable's value.
    */
-  public addEnvironment(key: string, value: any): this {
-    if (!this.environment) {
-      // TODO: add metadata
-      return this;
-    }
+  public addEnvironment(key: string, value: string): this {
     this.environment[key] = value;
     return this;
   }