[Bug #21838]

When we have allocatable slots, we can grow the heap instead of forcing
a major GC. This prevents major GC to be ran very often in certain situations.
See the ticket for more details.

On ruby-bench, we can see that this patch doesn't cause any significant
regressions:

    --------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------
    bench           master (ms)  stddev (%)  RSS (MiB)  branch (ms)  stddev (%)  RSS (MiB)  branch 1st itr  master/branch
    activerecord    148.2        0.3         59.2       150.0        0.8         69.7       1.015           0.988
    chunky-png      435.2        0.3         72.9       438.8        0.1         66.7       0.993           0.992
    erubi-rails     733.8        1.2         118.7      704.8        0.2         98.3       1.077           1.041
    hexapdf         1400.4       1.1         247.0      1405.0       0.9         223.7      0.986           0.997
    liquid-c        32.5         3.3         32.8       32.5         2.1         30.7       1.042           0.999
    liquid-compile  31.0         1.7         35.1       33.4         3.9         32.8       0.938           0.928
    liquid-render   84.7         0.4         30.8       86.3         0.4         30.8       0.981           0.982
    lobsters        594.7        0.6         310.5      596.6        0.4         306.0      1.057           0.997
    mail            75.6         2.8         53.3       76.9         0.7         53.2       0.968           0.982
    psych-load      1122.8       1.2         29.2       1145.1       0.4         31.7       0.964           0.981
    railsbench      1244.7       0.3         115.5      1254.8       1.1         115.2      0.939           0.992
    rubocop         103.7        0.5         94.1       104.3        0.5         92.4       0.985           0.994
    ruby-lsp        88.3         0.6         78.5       88.5         1.2         77.9       0.992           0.997
    sequel          26.9         0.9         33.6       28.3         1.4         32.1       0.954           0.952
    shipit          1119.3       1.5         171.4      1075.7       2.1         162.5      1.873           1.040
    --------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------