Investigate using indexmap instead of BTreeMap and Vec where now the .iter().position(...) is used for linear lookup

[mtolmacs]

Background

The indexmap crate is already used by the dependencies of this lib (wasmparser -> object) and would offer a better lookup speed while sparing the extra sorting on the data set, which can offer some speedup in sourcemap generation.

Success

No loss of performance. The usage of indexmap is more idiomatic in this setting, which is a win in itself.