Inline tables should be immutable

[moorereason]

Environment

toml++ version and/or commit hash:
v3 b41e12f

Compiler:
gcc 11.1.0

C++ standard mode:
c++20

Target arch:
linux x86_64

Library configuration overrides:
n/a

Relevant compilation flags:
meson build_tt --buildtype=release -Dbuild_tt_encoder=true -Dbuild_tt_decoder=true -Dgenerate_cmake_config=false

Describe the bug

The following TOML document should be invalid:

a={}
[a.b]  # INVALID

The TOML spec:

Inline tables are fully self-contained and define all keys and sub-tables within them. Keys and sub-tables cannot be added outside the braces.

Steps to reproduce (or a small repro code sample)

On Linux:

$ echo "a={}\n[a.b]" | tt_decoder
{"a":{"b":{}}}

Additional information

I've submitted a PR to toml-test for this case: toml-lang/toml-test#109

Found while doing differential fuzzing against go-toml.

[marzer]

Ah, this one is annoying. Initially this behaviour was correct, but was changed per a discussion on toml-lang/toml where others insisted I was incorrect to prohibit it. This was from before that snippet was added to the spec so I guess retroactively I was right all along hah

[marzer]

Fixed in v3, thanks!