Remove numerous things deprecated during the 0.14 release cycle

Basically everything that has a direct replacement or no uses left.

Notable omissions:

- std.ArrayHashMap: Too much fallout, needs a separate cleanup.
- std.debug.runtime_safety: Too much fallout.
- std.heap.GeneralPurposeAllocator: Lots of references to it remain, not
  a simple find and replace as "debug allocator" is not equivalent to
  "general purpose allocator".
- std.io.Reader: Is being reworked at the moment.
- std.unicode.utf8Decode(): No replacement, needs a new API first.
- Manifest backwards compat options: Removal would break test data used
  by TestFetchBuilder.
- panic handler needs to be a namespace: Many tests still rely on it
  being a function, needs a separate cleanup.

tools/docgen.zig

@@ -10,7 +10,7 @@ const mem = std.mem;
 const testing = std.testing;
 const Allocator = std.mem.Allocator;
 const getExternalExecutor = std.zig.system.getExternalExecutor;
-const fatal = std.zig.fatal;
+const fatal = std.process.fatal;
 
 const max_doc_file_size = 10 * 1024 * 1024;
 

tools/doctest.zig

@@ -1,6 +1,6 @@
 const builtin = @import("builtin");
 const std = @import("std");
-const fatal = std.zig.fatal;
+const fatal = std.process.fatal;
 const mem = std.mem;
 const fs = std.fs;
 const process = std.process;

tools/migrate_langref.zig

@@ -7,7 +7,7 @@ const mem = std.mem;
 const testing = std.testing;
 const Allocator = std.mem.Allocator;
 const max_doc_file_size = 10 * 1024 * 1024;
-const fatal = std.zig.fatal;
+const fatal = std.process.fatal;
 
 pub fn main() !void {
     var arena_instance = std.heap.ArenaAllocator.init(std.heap.page_allocator);