Enable the already implemented Quadratic Probing algorithm. It was introduced
in the following commit, alghough not enabled (it was not even compiled):
7577515

If we call `h(x)` to the hash function which maps `x` to a position in the
hash, then the i-th probe position for `x` is given by the following function:
`h(x,i) = h(x) + 1/2i + 1/2i^2`

This produces the following probe sequence:
`h(x), h(x) + 1, h(x) + 3, h(x) + 6, h(x) + 10, ...`
In this sequence the increase values are triangular numbers, making it
efficient to implement.

Quadratic Probing may perform better than the current enabled implementation
with Tabulation Hashing, which I plan to implement next.

Implement and enable Linear Probing. Keep the other two implementations under
the macro `PROBING`.

If we call `h(x)` to the hash function which maps `x` to a position in the
hash, then the i-th probe position for `x` is given by the following function:
`h(x,i) = h(x) + i`
In other words, if a collision occurs, we try the next hash cell until we find
one empty.

Linear Probing may perform better than the two other implementations with
Tabulation Hashing, which I plan to implement next.

In spite of its simple implementation, Simple Tabulation provides many desired
guarantees, such as linear probing. Reference:
    Thorup, Mikkel. "Fast and powerful hashing using tabulation."
    Communications of the ACM 60.7 (2017): 94-101.
    https://dl.acm.org/doi/abs/10.1145/3068772

This is just a proof of concept to figure out if Simple Tabulation could
improve the performance of the current Ruby Hashing implementation. The code is
not in the correct place and it is only implemented for 64 bits and for the
general case (excluding strings, numbers and symbols). But this is enough to
prove the potential of Simple Tabulation. Below are the results got executing
the following code:
https://gist.github.com/Ana06/cc6f3a737ad16548996a22718de8b01b

Time to add 600000 objects to an empty hash 100 times:
    - master (without Simple Tabulation): 29.68
    - master with Linear Probing (without Simple Tabulation): 99.76
    - master with Quadratic Probing (without Simple Tabulation): 29.97
    - master with Simple Tabulation: 15.76
    - master with Linear Probing and Simple Tabulation: 24.23
    - master with Quadratic Probing and Simple Tabulation: 13.92

The results are in seconds. `master` refers to ruby 2.8.0dev
(2020-05-07T16:22:38Z master 7ded8fd) [x86_64-linux].
I tried with 8 x Intel i5-8265U at 1.60 GHz.