toml: rename from tomlt, split raw AST into Value submodule

Drops the "t" suffix and follows the value/codec/toml/core pattern
(jsont.json_base style). The internal raw TOML module moves from
[Toml] to [Value] (file: lib/value.ml, was lib/toml.ml) to make room
for the top-level Toml facade (file: lib/toml.ml, was lib/tomlt.ml).

External callers now reach the raw AST through [Toml.Value.X] instead
of [Tomlt.Toml.X]. Every downstream reference updated in lockstep.