Add PYCRYPTODOME_DISABLE_GMP flag and simplify to_bytes() for GMP #845

Job	Run time
test_c_linux (0, x64)	52s
build_python_27 (ubuntu-20.04, yes)	42s
test_c_linux (0, x32)	1m 2s
mypy	14s
build_python_27 (ubuntu-20.04, no)	37s
test_c_linux (1, x64)	1m 13s
build (3.5, yes, ubuntu-20.04)	2m 21s
build_python_27 (windows-latest, yes)	46s
test_c_linux (1, x32)	1m 3s
build (3.5, no, ubuntu-20.04)	2m 11s
test_c_windows (x64)	55s
build_python_27 (windows-latest, no)	39s
build (3.6, yes, ubuntu-20.04)	2m 25s
test_c_windows (win32)	1m 2s
build (3.6, no, ubuntu-20.04)	2m 19s
build (3.7, yes, ubuntu-20.04)	2m 19s
build (3.7, no, ubuntu-20.04)	2m 13s
build (3.8, yes, ubuntu-20.04)	2m 8s
build (3.8, no, ubuntu-20.04)	2m 5s
build (3.9, yes, ubuntu-20.04)	2m 7s
build (3.9, no, ubuntu-20.04)	2m 1s
build (3.10, yes, ubuntu-20.04)	2m 2s
build (3.10, no, ubuntu-20.04)	2m 1s
build (3.11, yes, ubuntu-20.04)	1m 48s
build (3.11, no, ubuntu-20.04)	1m 52s
build (3.12-dev, yes, ubuntu-20.04)	1m 45s
build (3.12-dev, no, ubuntu-20.04)	1m 56s
build (3.11, yes, macos-12)	2m 28s
build (3.11, no, windows-latest)	4m 7s
build (3.11, yes, windows-latest)	4m 26s
build (3.12-dev, no, windows-latest)	4m 15s
build (3.12-dev, yes, windows-latest)	3m 53s
build (pypy2.7, no, ubuntu-latest)	1m 31s
build (pypy3.9, no, ubuntu-latest)	1m 37s
	1h 4m 55s

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