resolve compiler warnings: Deprecated, HoleEnumConv

[dctucker]

This makes the following warnings go away when compiling.

~/Development/nimgl/src/nimgl/glfw.nim(1051, 18) Warning: use `char` or `uint8` instead; cuchar is deprecated [Deprecated]
~/Development/nimgl/src/nimgl/glfw.nim(1488, 50) Warning: conversion to enum with holes is unsafe: GLFWKey(x) [HoleEnumConv]
~/Development/nimgl/src/nimgl/glfw.nim(1490, 50) Warning: conversion to enum with holes is unsafe: GLFWHat(x) [HoleEnumConv]
~/Development/nimgl/src/nimgl/glfw.nim(4487, 65) Warning: use `char` or `uint8` instead; cuchar is deprecated [Deprecated]
~/Development/nimgl/src/nimgl/glfw.nim(4526, 62) Warning: use `char` or `uint8` instead; cuchar is deprecated [Deprecated]
~/Development/nimgl/src/nimgl/glfw.nim(5427, 42) Warning: use `char` or `uint8` instead; cuchar is deprecated [Deprecated]

• Adding a bunch of enum values to fill in the holes in GLFWKey, and a few to GLFWHat.
• cuchar becomes uint8