Add methods and types for reading from `MeshBuilder`

[ColonelThirtyTwo]

MeshBuilder is a convenient way of storing mesh data on the CPU, since it supports a wide variety of formats due to using arbitrary vertex attributes. However it does not have any way to read the data stored inside of it.

This patch adds functionality for reading the mesh stored inside of the buffers:

• MeshBuilder::view_attr finds an attribute in the stored buffers by name and returns a view into the buffer that reads that attribute. It can also be iterated over.
• MeshBuffer::iter_index iterates over elements in the index buffer, or from 0..numvertices if there is no index buffer.
• Various getters for MeshBuffer and RawVertexBuffer
• FromVertexBuffer defines how to read a type from a vertex buffer and convert it from the vertex format its in (ex. normalizing and scaling integer values into floating-point).

Wrote this as part of amethyst/amethyst#2076