Pre-compile pack/unpack function calls

I noticed that pack/unpack functions from
https://github.com/dpkp/kafka-python/blob/master/kafka/protocol/types.py
might be slightly improved. I made pre-compilation for them. It gives
about 10% better performance compared to the current implementation.

Consumption of 100msg:
```
239884    0.187    0.000    0.287    0.000 types.py:18(_unpack) # new version
239884    0.192    0.000    0.323    0.000 types.py:17(_unpack)
```

I also made some profiling for producers/consumers. It gives about
1-1.5% time savings.

kafka/protocol/types.py

@@ -1,13 +1,14 @@
 from __future__ import absolute_import
 
-from struct import pack, unpack, error
+import struct
+from struct import error
 
 from kafka.protocol.abstract import AbstractType
 
 
 def _pack(f, value):
     try:
-        return pack(f, value)
+        return f(value)
     except error as e:
         raise ValueError("Error encountered when attempting to convert value: "
                         "{!r} to struct format: '{}', hit error: {}"
@@ -16,7 +17,7 @@ def _pack(f, value):
 
 def _unpack(f, data):
     try:
-        (value,) = unpack(f, data)
+        (value,) = f(data)
         return value
     except error as e:
         raise ValueError("Error encountered when attempting to convert value: "
@@ -25,43 +26,55 @@ def _unpack(f, data):
 
 
 class Int8(AbstractType):
+    _pack = struct.Struct('>b').pack
+    _unpack = struct.Struct('>b').unpack
+
     @classmethod
     def encode(cls, value):
-        return _pack('>b', value)
+        return _pack(cls._pack, value)
 
     @classmethod
     def decode(cls, data):
-        return _unpack('>b', data.read(1))
+        return _unpack(cls._unpack, data.read(1))
 
 
 class Int16(AbstractType):
+    _pack = struct.Struct('>h').pack
+    _unpack = struct.Struct('>h').unpack
+
     @classmethod
     def encode(cls, value):
-        return _pack('>h', value)
+        return _pack(cls._pack, value)
 
     @classmethod
     def decode(cls, data):
-        return _unpack('>h', data.read(2))
+        return _unpack(cls._unpack, data.read(2))
 
 
 class Int32(AbstractType):
+    _pack = struct.Struct('>i').pack
+    _unpack = struct.Struct('>i').unpack
+
     @classmethod
     def encode(cls, value):
-        return _pack('>i', value)
+        return _pack(cls._pack, value)
 
     @classmethod
     def decode(cls, data):
-        return _unpack('>i', data.read(4))
+        return _unpack(cls._unpack, data.read(4))
 
 
 class Int64(AbstractType):
+    _pack = struct.Struct('>q').pack
+    _unpack = struct.Struct('>q').unpack
+
     @classmethod
     def encode(cls, value):
-        return _pack('>q', value)
+        return _pack(cls._pack, value)
 
     @classmethod
     def decode(cls, data):
-        return _unpack('>q', data.read(8))
+        return _unpack(cls._unpack, data.read(8))
 
 
 class String(AbstractType):
@@ -108,13 +121,16 @@ def repr(cls, value):
 
 
 class Boolean(AbstractType):
+    _pack = struct.Struct('>?').pack
+    _unpack = struct.Struct('>?').unpack
+
     @classmethod
     def encode(cls, value):
-        return _pack('>?', value)
+        return _pack(cls._pack, value)
 
     @classmethod
     def decode(cls, data):
-        return _unpack('>?', data.read(1))
+        return _unpack(cls._unpack, data.read(1))
 
 
 class Schema(AbstractType):