Increase buffer indexing variable size

I looked over the users of these variables and they should be fine with
no additional changes. The existing methods already have an option to
use size_t rather than uint8_t.

There's a few methods which return int instead of size_t, which isn't
great from a portability perspective but will be fine since this only is
designed to run on the ESP8266.

libraries/Wire/Wire.cpp

@@ -41,13 +41,13 @@ extern "C" {
 // Initialize Class Variables //////////////////////////////////////////////////
 
 uint8_t TwoWire::rxBuffer[I2C_BUFFER_LENGTH];
-uint8_t TwoWire::rxBufferIndex = 0;
-uint8_t TwoWire::rxBufferLength = 0;
+size_t TwoWire::rxBufferIndex = 0;
+size_t TwoWire::rxBufferLength = 0;
 
 uint8_t TwoWire::txAddress = 0;
 uint8_t TwoWire::txBuffer[I2C_BUFFER_LENGTH];
-uint8_t TwoWire::txBufferIndex = 0;
-uint8_t TwoWire::txBufferLength = 0;
+size_t TwoWire::txBufferIndex = 0;
+size_t TwoWire::txBufferLength = 0;
 
 uint8_t TwoWire::transmitting = 0;
 void (*TwoWire::user_onRequest)(void);

libraries/Wire/Wire.h

@@ -38,13 +38,13 @@ class TwoWire : public Stream
 {
 private:
     static uint8_t rxBuffer[];
-    static uint8_t rxBufferIndex;
-    static uint8_t rxBufferLength;
+    static size_t rxBufferIndex;
+    static size_t rxBufferLength;
 
     static uint8_t txAddress;
     static uint8_t txBuffer[];
-    static uint8_t txBufferIndex;
-    static uint8_t txBufferLength;
+    static size_t txBufferIndex;
+    static size_t txBufferLength;
 
     static uint8_t transmitting;
     static void (*user_onRequest)(void);