Switch serialization format for variables and history

**fish version installed**:

``` sh
~/ref/tools/fish-shell/fish -v
fish, version 2.3.1-492-g3702616
```

**OS/terminal used**:

``` fish
$ uname -a
Darwin Joshs-MacBook-Air-2.local 15.6.0 Darwin Kernel Version 15.6.0: Thu Jun 23 18:25:34 PDT 2016; root:xnu-3248.60.10~1/RELEASE_X86_64 x86_64

$ sw_vers
ProductName: Mac OS X
ProductVersion: 10.11.6
BuildVersion: 15G31
```

**Issue:**

Variables do not get set when their value is exactly the character `\x1d`.
## Reproduction steps

``` fish
$ set -l oned \x1d
$ set -l onec \x1c

# We see that onec has a value, but oned does not
$ set -l
onec \x1c
oned
```