convert Patcher to a global singleton

Migrates Patcher state to global variables and uses std.once for initialization.
This is preparing for future syscall tracing, which requires static access to
the patching context across the runtime to be accessed by flicken.

src/main.zig

@@ -32,8 +32,6 @@ const help =
 
 const UnfinishedReadError = error{UnfinishedRead};
 
-var patcher: Patcher = undefined;
-
 pub fn main() !void {
     // Parse arguments
     var arg_index: u64 = 1; // Skip own name
@@ -52,10 +50,10 @@ pub fn main() !void {
     }
 
     // Initialize patcher
-    patcher = try Patcher.init(std.heap.page_allocator); // TODO: allocator
+    Patcher.init();
     // Block the first 64k to avoid mmap_min_addr (EPERM) issues on Linux.
     // TODO: read it from `/proc/sys/vm/mmap_min_addr` instead.
-    try patcher.address_allocator.block(patcher.gpa, .{ .start = 0, .end = 0x10000 }, 0);
+    try Patcher.address_allocator.block(Patcher.gpa, .{ .start = 0, .end = 0x10000 }, 0);
 
     // Map file into memory
     const file = try lookupFile(mem.sliceTo(std.os.argv[arg_index], 0));
@@ -207,7 +205,7 @@ fn loadStaticElf(ehdr: elf.Header, file_reader: *std.fs.File.Reader) !usize {
         const protections = elfToMmapProt(phdr.p_flags);
         if (protections & posix.PROT.EXEC > 0) {
             log.info("Patching executable segment", .{});
-            try patcher.patchRegion(ptr);
+            try Patcher.patchRegion(ptr);
         }
         try posix.mprotect(ptr, protections);
     }