add new float-parser based on eisel-lemire algorithm

The previous float-parsing method was lacking in a lot of areas. This
commit introduces a state-of-the art implementation that is both
accurate and fast to std.

Code is derived from working repo https://github.com/tiehuis/zig-parsefloat.
This includes more test-cases and performance numbers that are present
in this commit.

* Accuracy

The primary testing regime has been using test-data found at
https://github.com/tiehuis/parse-number-fxx-test-data. This is a fork of
upstream with support for f128 test-cases added. This data has been
verified against other independent implementations and represents
accurate round-to-even IEEE-754 floating point semantics.

* Performance

Compared to the existing parseFloat implementation there is ~5-10x
performance improvement using the above corpus. (f128 parsing excluded
in below measurements).

** Old

    $ time ./test_all_fxx_data
    3520298/5296694 succeeded (1776396 fail)

    ________________________________________________________
    Executed in   28.68 secs    fish           external
       usr time   28.48 secs    0.00 micros   28.48 secs
       sys time    0.08 secs  694.00 micros    0.08 secs

** This Implementation

    $ time ./test_all_fxx_data
    5296693/5296694 succeeded (1 fail)

    ________________________________________________________
    Executed in    4.54 secs    fish           external
       usr time    4.37 secs  515.00 micros    4.37 secs
       sys time    0.10 secs  171.00 micros    0.10 secs

Further performance numbers can be seen using the
https://github.com/tiehuis/simple_fastfloat_benchmark/ repository, which
compares against some other well-known string-to-float conversion
functions. A breakdown can be found here:

https://github.com/tiehuis/zig-parsefloat/blob/0d9f020f1a37ca88bf889703b397c1c41779f090/PERFORMANCE.md#commit-b15406a0d2e18b50a4b62fceb5a6a3bb60ca5706

In summary, we are within 20% of the C++ reference implementation and
have about ~600-700MB/s throughput on a Intel I5-6500 3.5Ghz.

* F128 Support

Finally, f128 is now completely supported with full accuracy. This does
use a slower path which is possible to improve in future.

* Behavioural Changes

There are a few behavioural changes to note.

 - `parseHexFloat` is now redundant and these are now supported directly
   in `parseFloat`.
 - We implement round-to-even in all parsing routines. This is as
   specified by IEEE-754. Previous code used different rounding
   mechanisms (standard was round-to-zero, hex-parsing looked to use
   round-up) so there may be subtle differences.

Closes ziglang#2207.
Fixes ziglang#11169.

lib/std/fmt.zig

@@ -1837,8 +1837,8 @@ test "parseUnsigned" {
 }
 
 pub const parseFloat = @import("fmt/parse_float.zig").parseFloat;
+pub const parseHexFloat = @compileError("deprecated; use `parseFloat`");
 pub const ParseFloatError = @import("fmt/parse_float.zig").ParseFloatError;
-pub const parseHexFloat = @import("fmt/parse_hex_float.zig").parseHexFloat;
 
 test {
     _ = parseFloat;