fix: escape escape codes in the serializer

[0x5a4]

fixes #21

I've also noticed another issue which I fixed alongside this. Previously, when given a string like hello\n we would happily write this to the output stream:

key = "hello
"

which is of course very wrong.

[0x5a4]

hmmm, how should we handle non ascii chars? should we just write them as is (the current behaviour)? or maybe convert them to the appropriate toml escape code \uXXXX

[mattyhall]

hmmm, how should we handle non ascii chars?

The docs say:

Any Unicode character may be used except those that must be escaped: quotation mark, backslash, and the control characters other than tab (U+0000 to U+0008, U+000A to U+001F, U+007F).

So I think we can leave them as is, apart from the control characters which we do need to escape.

[mattyhall]

Thanks!

[0x5a4]

could you merge #17 as well?