Remove excessive bounds checking in IndexedReader

[drewnoakes]

Fixes #62

Adds a new abstract overload for reading a single byte. That method will not do any explicit bounds validation, so callers must do that beforehand. In cases where multiple bytes are read, the validation can be performed once per value, rather than once per byte.

[drewnoakes]

This has some failures in the (manual) regression tests that need to be investigated.