Add missing fmpz_mod_mpoly_compose_ functions #2825

Job	Run time
macOS-x86 Clang with BLAS (x2)	16m 33s
macOS-M1, Clang (x3)	8m 7s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 6s
GCC build regression check (no tests)	1m 5s
MinGW GCC (x0.5)	10m 30s
MSVC (x1)	9m 43s
Nemo.jl (temporary branch)	12m 0s
Ubuntu Clang with examples (x5)	12m 39s
Ubuntu GCC, Code Coverage (x10)	31m 1s
Ubuntu GCC with NTL (assert, x2)	8m 35s
	2h 4m 19s

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