token: Optimize zig version (#19)

#### Problem

There were a few issues holding back the zig version of the SPL Token.
There was an unnecessary copy in close account and a few checks that
could be moved.

#### Summary of changes

Use the newest SDK with a more efficient entrypoint. Optimize some stuff
and refactor the code for readability.

README.md

@@ -215,39 +215,39 @@ program.
 | Language | CU Usage |
 | --- | --- |
 | Rust | 1115 |
-| Zig | 166 |
+| Zig | 158 |
 
   * Initialize Account
 
 | Language | CU Usage |
 | --- | --- |
 | Rust | 2071 |
-| Zig | 189 |
+| Zig | 176 |
 
   * Mint To
 
 | Language | CU Usage |
 | --- | --- |
 | Rust | 2189 |
-| Zig | 195 |
+| Zig | 179 |
 
   * Transfer
 
 | Language | CU Usage |
 | --- | --- |
 | Rust | 2208 |
-| Zig | 160 |
+| Zig | 148 |
 
   * Burn
 
 | Language | CU Usage |
 | --- | --- |
 | Rust | 2045 |
-| Zig | 157 |
+| Zig | 145 |
 
   * Close Account
 
 | Language | CU Usage |
 | --- | --- |
 | Rust | 1483 |
-| Zig | 260 |
+| Zig | 130 |

token/zig/build.zig.zon

@@ -16,8 +16,8 @@
     // internet connectivity.
     .dependencies = .{
         .@"solana-program-sdk" = .{
-            .url = "https://github.com/joncinque/solana-program-sdk-zig/archive/refs/tags/v0.15.0.tar.gz",
-            .hash = "1220c255d7d80a59251d901da4d2982eb660d099680c1207b14f51078987c655c979",
+            .url = "https://github.com/joncinque/solana-program-sdk-zig/archive/refs/tags/v0.15.1.tar.gz",
+            .hash = "12203631b9eba91c479991ec8f0525f181addb5879bbb96e256427f802c2ca67e108",
         },
     },