Add freelist for Decimal's

[skirpichev]

Benchmark	ref	patch
decimal_factorial	1.06 sec	974 ms: 1.08x faster
decimal_pi	2.00 sec	1.83 sec: 1.10x faster
Geometric mean	(ref)	1.09x faster

(with ./configure -q)

PGO:

Benchmark	ref	patch
decimal_factorial	964 ms	923 ms: 1.04x faster
decimal_pi	1.66 sec	1.58 sec: 1.05x faster
Geometric mean	(ref)	1.05x faster

[skirpichev]

At 8c935e3 (with ./configure -q):

decimal_factorial: Mean +- std dev: [ref] 1.01 sec +- 0.01 sec -> [fl] 934 ms +- 4 ms: 1.08x faster
decimal_pi: Mean +- std dev: [ref] 1.99 sec +- 0.03 sec -> [fl] 1.71 sec +- 0.01 sec: 1.17x faster

Geometric mean: 1.12x faster