This is to differentiate between the upcoming `AllocatingMemoryStream`,
which automatically allocates memory as needed instead of operating on a
static memory area.
This generally seems like a better name, especially if we somehow also
need a better name for "read the entire buffer, but not the entire file"
somewhere down the line.
LibFuzzer documentation [1] states that all return values except for 0
and -1 are currently reserved for future use. -1 is a special return
value that causes LibFuzzer to not add a testing input to the testing
corpus, regardless of the code coverage that it causes.
[1] https://llvm.org/docs/LibFuzzer.html
