refactor: extract modules from bot.zig

This commit creates a bunch of new modules that contain code and tests
for various concepts/implementations that used to exist inside
bot.zig.

Notable amongst these are:
- buffer.zig, which contain the circular buffer containing both
  backlog and outbox messages.
- parser.zig, which contain the parser used to parse commands from IRC
  messages.

src/parser.zig

@@ -0,0 +1,54 @@
+const std = @import("std");
+
+pub fn init(s: []const u8) Parser {
+    return .init(s);
+}
+
+pub const Parser = struct {
+    original: []const u8,
+    rest: []const u8,
+    end_idx: usize,
+
+    pub fn seek(self: *const Parser, skip: usize) Parser {
+        return .{ .original = self.original, .rest = self.rest[skip..], .end_idx = self.end_idx + skip };
+    }
+
+    pub fn init(s: []const u8) Parser {
+        return .{
+            .original = s,
+            .end_idx = 0,
+            .rest = s,
+        };
+    }
+
+    pub fn consume_char(self: *const Parser, c: u8) ?Parser {
+        if (self.rest[0] != c) {
+            return null;
+        }
+        return self.seek(1);
+    }
+
+    pub fn consume_str(self: *const Parser, s: []const u8) ?Parser {
+        const len = s.len;
+        if (self.rest.len < len) {
+            return null;
+        }
+        if (!std.mem.eql(u8, self.rest[0..len], s)) {
+            return null;
+        }
+        return self.seek(len);
+    }
+
+    pub fn take_until_char(self: *const Parser, c: u8) struct { Parser, []const u8 } {
+        const idx = std.mem.indexOfScalar(u8, self.rest, c) orelse unreachable;
+        return .{ self.seek(idx), self.rest[0..idx] };
+    }
+
+    pub fn take_char(self: *const Parser) struct { Parser, u8 } {
+        return .{ self.seek(1), self.rest[0] };
+    }
+
+    pub fn parsed(self: *const Parser) []const u8 {
+        return self.original[0..self.end_idx];
+    }
+};