Add missing fmpz_mod_mpoly_compose_ functions #2805

Job	Run time
macOS-x86 Clang with BLAS (x2)	15m 19s
GCC build regression check (no tests)	1m 2s
macOS-M1, Clang (x3)	8m 43s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 11s
MinGW GCC (x0.5)	10m 35s
Nemo.jl (temporary branch)	11m 57s
MSVC (x1)	9m 31s
Ubuntu Clang with examples (x5)	12m 34s
Ubuntu GCC, Code Coverage (x10)	29m 59s
Ubuntu GCC with NTL (assert, x2)	8m 24s
	2h 2m 15s

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