A convenience class for reading the contents of a file into a
byte. This class attempts to minimize the number of allocations
and copies required to read this data.
For the case where we know the "true" length of a file (most ordinary files)
we allocate exactly one byte and copy data into that. Calls to
#toByteArray will then return the internal array and not a copy.
Note that an absolute path must be supplied. Expect your reads to fail
if one isn't.