stage2: tokenizer: require null terminated source

By requiring the source file to be null-terminated, we avoid extra
branching while simplifying the logic at the same time.

Running ast-check on a large zig source file (udivmodti4_test.zig),
master branch compared to this commit:
 * 4% faster wall clock
 * 7% fewer cache misses
 * 1% fewer branches

lib/std/zig/parse.zig

@@ -17,7 +17,7 @@ pub const Error = error{ParseError} || Allocator.Error;
 
 /// Result should be freed with tree.deinit() when there are
 /// no more references to any of the tokens or nodes.
-pub fn parse(gpa: *Allocator, source: []const u8) Allocator.Error!Tree {
+pub fn parse(gpa: *Allocator, source: [:0]const u8) Allocator.Error!Tree {
     var tokens = ast.TokenList{};
     defer tokens.deinit(gpa);