Cesium should support XML documentation

Cesium types and functions should allow documentation in `///` style, similar to C# or F#.

E.g.
```
///
/// The full value is mantissa / (10 ^ places)
///
typedef struct Decimal {
uint8_t places;
uint32_t mantissaLength;
uint8_t *mantissa;
} Decimal;
```

This should generate the `Assembly.xml` documentation file containing the doc for the type `Decimal`.

Start by tracing how Cesium parses `///` comments and emits the `Assembly.xml` documentation file. Use the Decimal example as the expected input and verify that the generated file contains documentation for the Decimal type, including the summary text.

Written by the indexing model from the issue text.