Protect against circular type forwarders

[KirillOsenkov]

If we have two assemblies with type forwarders that point to each other, we enter an infinite loop and a stack overflow.

This breaks the cycle by detecting reentrancy.

Fixes #706

[KirillOsenkov]

@jbevain I even added a test!

[jonlouie]

@jbevain What can community contributors do to get this PR merged and released? This resolves a bug that my team encountered.

[KirillOsenkov]

FYI @jbevain adding the boolean field is free because of alignment padding:

[KirillOsenkov]

Unfortunately it's not free for 32-bit (the size goes from 32 to 36 bytes. But there are not many exported types per assembly, so it's honestly negligible.

Type layout for 'ExportedType'
Size: 36 bytes. Paddings: 3 bytes (%8 of empty space)
|==============================================|
| Object Header (4 bytes)                      |
|----------------------------------------------|
| Method Table Ptr (4 bytes)                   |
|==============================================|
|   0-3: String namespace (4 bytes)            |
|----------------------------------------------|
|   4-7: String name (4 bytes)                 |
|----------------------------------------------|
|  8-11: IMetadataScope scope (4 bytes)        |
|----------------------------------------------|
| 12-15: ModuleDefinition module (4 bytes)     |
|----------------------------------------------|
| 16-19: ExportedType declaring_type (4 bytes) |
|----------------------------------------------|
| 20-23: UInt32 attributes (4 bytes)           |
|----------------------------------------------|
| 24-27: Int32 identifier (4 bytes)            |
|----------------------------------------------|
|    28: Boolean reentrancyGuard (1 byte)      |
|----------------------------------------------|
| 29-31: padding (3 bytes)                     |
|----------------------------------------------|
| 32-35: MetadataToken token (4 bytes)         |
| |===============================|            |
| |   0-3: UInt32 token (4 bytes) |            |
| |===============================|            |
|==============================================|