Add 'Read*' deserialize methods

[agocke]

This heavily improves performance by avoiding generic virtual dispatch in the common case of deserializing primitive types inside a struct.

After these changes, the performance difference between Serde and S.T.J is almost entirely eliminated:

| Method      | Mean       | Error   | StdDev  | Gen0   | Gen1   | Allocated |
|------------ |-----------:|--------:|--------:|-------:|-------:|----------:|
| JsonNet     | 1,032.3 ns | 2.80 ns | 2.48 ns | 0.4063 | 0.0019 |    3400 B |
| SystemText  |   617.0 ns | 3.14 ns | 2.62 ns | 0.0534 |      - |     448 B |
| SerdeJson   |   633.4 ns | 1.44 ns | 1.21 ns | 0.0963 |      - |     808 B |
| SerdeManual |   632.7 ns | 4.01 ns | 3.13 ns | 0.0963 |      - |     808 B |