Allow printing gr_ctx; extend documentation; hopefully fix clang #275

Job	Run time
GCC build regression check (no tests)	1m 5s
macOS-M1, Clang (x3)	8m 3s
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 57s
MinGW GCC (x0.5)	10m 24s
MSVC (x1)	9m 34s
Nemo.jl	18m 7s
Ubuntu Clang, BLAS and examples (x5)	12m 27s
Ubuntu GCC, Code Coverage (x10)	29m 50s
Ubuntu GCC with NTL (assert, x2)	9m 4s
	1h 52m 31s

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