Add way to access unknown custom sections in `Module`

[Robbepop]

Currently Wasmi just ignores Wasm custom sections when parsing a Wasm module.
However, some users might have a need for querying information about their own custom sections.

For this purpose we might want to add a way to access and query a Wasm modules custom section information.
A potential API for this might look like the following:

impl Module {
    pub fn custom_sections(&self) -> impl Iterator<Item = CustomSection>;
}

struct CustomSection { ... }

impl CustomSection {
    pub fn name(&self) -> &str { ... }
    pub fn data(&self) -> &[u8] { ... }
}

Once Wasmi supports the Wasm standardized name and branch hints custom sections these should no longer be yielded by Module::custom_sections.

In order to not pessimize performance when parsing or validating Wasm modules with custom sections compared to today's Wasmi we might want to add a flag to wasmi::Config to indicate that a user is not at all interested in custom section information and disable the new functionality entirely. This new wasmi::Config flag might be set using config.ignore_custom_sections(true). If this flag is set to true then Module::custom_sections always returns an iterator that yields no items even if custom sections have been present in the Wasm module.

tl;dr

• Add Module::custom_sections method
• Do not yield known Wasm custom sections in Module::custom_sections such as name and branch hints section
• Add Config::ignore_custom_sections configuration