perf: remove jwk2privPem and jwk2pubPem (#162)

These 2 unused functions required us to import the whole of the node-forge PKI implementation when we only use some RSA stuffs.

BREAKING CHANGE: removes unused jwk2pem methods `jwk2pubPem` and `jwk2privPem`. These methods are not being used in any js libp2p modules, so only users referencing these directly will be impacted.

src/keys/jwk2pem.js

@@ -1,10 +1,9 @@
 'use strict'
 
-const forge = {
-  util: require('node-forge/lib/util'),
-  pki: require('node-forge/lib/pki'),
-  jsbn: require('node-forge/lib/jsbn')
-}
+require('node-forge/lib/rsa')
+require('node-forge/lib/util')
+require('node-forge/lib/jsbn')
+const forge = require('node-forge/lib/forge')
 
 function base64urlToBigInteger (str) {
   var bytes = forge.util.decode64(
@@ -22,21 +21,11 @@ function jwk2priv (key) {
   return forge.pki.setRsaPrivateKey(...convert(key, ['n', 'e', 'd', 'p', 'q', 'dp', 'dq', 'qi']))
 }
 
-function jwk2privPem (key) {
-  return forge.pki.privateKeyToPem(jwk2priv(key))
-}
-
 function jwk2pub (key) {
   return forge.pki.setRsaPublicKey(...convert(key, ['n', 'e']))
 }
 
-function jwk2pubPem (key) {
-  return forge.pki.publicKeyToPem(jwk2pub(key))
-}
-
 module.exports = {
   jwk2pub,
-  jwk2pubPem,
-  jwk2priv,
-  jwk2privPem
+  jwk2priv
 }