Add missing fmpz_mod_mpoly_compose_ functions #2820

Job	Run time
macOS-M1, Clang (x3)	8m 14s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 11s
GCC build regression check (no tests)	1m 2s
macOS-x86 Clang with BLAS (x2)	15m 19s
MinGW GCC (x0.5)	10m 47s
MSVC (x1)	9m 22s
Nemo.jl (temporary branch)	12m 6s
Ubuntu Clang with examples (x5)	12m 47s
Ubuntu GCC, Code Coverage (x10)	29m 53s
Ubuntu GCC with NTL (assert, x2)	8m 32s
	2h 2m 13s

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