Slices: added Big-Endian versions of method to read and write fixed-size 16, 32 and 64 integers, and helpers to create slices filled with a specific byte pattern

- Ex: WriteFixed16(...) will use Little-Endian (does not sort lexicographically) while WriteFixed16BE(...) will use Big-Endian (does sort lexicographically).
- Slice.Repeat(count, byte) creates a slice filled with a specific byte.
- Added SliceHelpers.SetBytes that uses memset() when possible.
Performance: don't mark private or public Slice fields as readonly, to reduce the number of copies on the stack

- See Jon Skeet's blog about this issue: http://msmvps.com/blogs/jon_skeet/archive/2014/07/16/micro-optimization-the-surprising-inefficiency-of-readonly-fields.aspx
- tl;dr: any readonly struct is first copied on the stack when you invoke a method or property on it (enven if it is immutable)
- readonly Slice are everywhere and called a lot, especially in the SliceReaders and Subspaces :(
- Made all 'public readonly Slice' into a private backing field
- added '//PERF: readonly struct' comment marking, until there is a better way to mark these fields somehow.
Updated unit test to reflect the fact that a memory leak was fixed in fdb_c v2.0.8

- native memory is now properly cleared when tr.Reset() is called