Currently, whenever we want to serialize an enum,
we simply serialize the underlying type (byte, short, int)
This works fine, but in order to get the reader and writer
at runtime I need a function for each type.
With this PR, we generate a reader and writer function for enums too,
the function body simply calls the underlying reader and writer.
* throwing Exception instead of returning null
* re-adding null check until later PR
* removing error from test
* removing extra error
* adding comment
* replacing error with throw for abstact
The plan is to remove WeaverLists at some point so moving the functions
out of weaver to here for now and then move them closer to where they
are actaully used when we start to remove WeaverLists.
# Before
This is hard to read:
```
if (ca.AttributeType.FullName == typeof(Mirror.CommandAttribute).FullName) {
...
}
```
# After
```
if (ca.AttributeType.Is<Mirror.CommandAttribute>()) {
...
}
```
# Before
WeaverTypes declared a static variable like this:
```cs
public static TypeReference int32Type;
```
Weavertypes then loaded the variable in SetupTargetTypes like this:
```cs
int32Type = ImportSystemModuleType(currentAssembly, systemModule, "System.Int32");
```
Note we lookup the type with a string, any typo there won't be detected until the code is executed. It will also not be found by the IDE when you click "find references", or when you refactor.
Then we use it like this:
```cs
var intType = WeaverTypes.int32Type;
```
This is a DRY violation: I have to modify 3 different code sections just to say that I want the int32 type.
# After
Get rid of all the duplication, and use the type instead of a string:
```cs
var intType = WeaverTypes.Import<int>();
```
No need for static variable, or to add it in SetupTargetTypes.
