erfinv(::BigFloat) enters infinite loop

What the title says. The problem seems to be with the Newton's method implementation in lines 432-437 of `erf.jl`.
```julia
while true # Newton iterations
Δx = sqrtπhalf * (erf(x) - y) * exp(x^2)
x -= Δx
abs(Δx) < tol && break
i += 1
end
```

I changed the loop to a finite one and logged the value if the loop ran 10000 iterations. The following vector of logged values should be useful for testing; I tried out a few of them and I indeed got stuck in an infinite loop.

```julia
testvals = BigFloat[
0.9395615430047861149631671651150099933147430419921875
0.9404791642273144791630556937889195978641510009765625
0.941154782589318283925194918992929160594940185546875
0.941194121590200882820909100701101124286651611328125
0.9412980378578670315192766793188638985157012939453125
0.941392615179371006206565652973949909210205078125
0.941392812046334181985685063409619033336639404296875
0.94139837911296542216632587951608002185821533203125
0.9414025544129389633241089541115798056125640869140625
0.9415072243757318659618249512277543544769287109375
0.94273997012758226077266954234801232814788818359375
0.94381178738070303779750247485935688018798828125
0.943972361330209341900854269624687731266021728515625
0.944223446113291675629852761630900204181671142578125
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]
```