Improve decoding performance

This yields a performance improvement over indexing into `source` over and over. `source` is asserted to be a string, `psz` is monotonically increasing, and `psz >= source.length` is a good indication that you've run out of characters.

Originally proposed by @oscxc in cryptocoinjs#74.

src/index.js

@@ -80,7 +80,7 @@ function base (ALPHABET) {
     var size = (((source.length - psz) * FACTOR) + 1) >>> 0 // log(58) / log(256), rounded up.
     var b256 = new Uint8Array(size)
         // Process the characters.
-    while (source[psz]) {
+    while (psz < source.length) {
             // Decode character
       var carry = BASE_MAP[source.charCodeAt(psz)]
             // Invalid character

ts_src/index.ts

@@ -99,7 +99,7 @@ function base (ALPHABET: string): base.BaseConverter {
     const b256 = new Uint8Array(size)
 
     // Process the characters.
-    while (source[psz]) {
+    while (psz < source.length) {
       // Decode character
       let carry = BASE_MAP[source.charCodeAt(psz)]