16-bit float support

[dalle]

Potential fix for issue #148
Augmenting initial work done in pull request #264

Still some issues remain.

• One test is failing:

Checking data/ulfjack-ryu.txt
fesetround to FE_UPWARD
bad 16: 0000 33000000 3E60000000000000 2.98023223876953125E-8
parsed as 0x1p-24 (5.9604644775390625e-08)
as raw uint16_t, parsed = 1, expected = 0
fesetround: FE_UPWARD

• Cleaning of code
• More tests and cleaning up tests
• Update readme

[dalle]

I would argue that the test is incorrect, parsing 2.98023223876953125E-8 (0.0000000298023223876953125) is exactly halfway to 5.9604644775390625E-8 (0.0000000596046447753906250) and would round upwards for fesetround: FE_UPWARD.

[lemire]

@dalle Thanks. I will have a look shortly.

Note that I do not trust the tests that I had included. We need to validate.