[WIP] enable randn(::BigFloat) and randexp(::BigFloat) (implemented in MPFR)

[rfourquet]

Closes #17629.

[rfourquet]

This currently doesn't support immutable RNGs. I tried the approach of wrapping an immutable RNG inside a mutable one, but this segfaults. Ideas welcome :)

[ViralBShah]

Would be nice to revive this and get it done.

[rfourquet]

This is now updated. It doesn't segfault anymore with immutable RNGs, but I had to add a special case for TaskLocalRNG, as it otherwise segfaults with this error:

signal (11): Segmentation fault
in expression starting at REPL[18]:1
typekeyvalue_hash at /local/src/julia/src/jltypes.c:1167 [inlined]
lookup_typevalue at /local/src/julia/src/jltypes.c:725
jl_inst_arg_tuple_type at /local/src/julia/src/jltypes.c:1603
arg_type_tuple at /local/src/julia/src/gf.c:1894 [inlined]
jl_lookup_generic_ at /local/src/julia/src/gf.c:2493 [inlined]
ijl_apply_generic at /local/src/julia/src/gf.c:2545
_randget_fn at /local/src/julia/usr/share/julia/stdlib/v1.9/Random/src/normal.jl:185
unknown function (ip: 0x7f134fa5f34c)
__gmp_urandomb_ui at /home/rf/.nix-profile/lib/libgmp.so (unknown line)
random_deviate_generate at /home/rf/.nix-profile/lib/libmpfr.so (unknown line)
mpfr_random_deviate_tstbit at /home/rf/.nix-profile/lib/libmpfr.so (unknown line)
mpfr_nrandom at /home/rf/.nix-profile/lib/libmpfr.so (unknown line)
rand at /local/src/julia/usr/share/julia/stdlib/v1.9/Random/src/normal.jl:224
[...]

I don't understand this error of course... This also means that the segfault would probably occur with custom RNGs which wrap a TaskLocalRNG, so we would have to warn to not do that, but this is really not ideal.