Chiptool failing on names starting with a number

Some SVDs (e.g. for the TI CC2650 MCU) contain names starting with numbers (in this case '32CYC'). Chiptool tries to create `Ident`s with such names and fails as they are not valid rust.

Proposed solution: if name starts with a number, prepend the `Ident` name with "_".

Start by locating where chiptool creates Rust Idents from SVD names, then reproduce the failure with the TI CC2650 SVD and the `32CYC` name. Done means names beginning with a number produce valid Rust identifiers by receiving the proposed leading underscore, without changing other names.

Written by the indexing model from the issue text.