make vector/matrix component length() static

[MaliusArth]

length() returns a constant which is specific to a type so no need to rely on an instance,
the need to create a temporary instance e.g in template functions is annoying and unnecessary
see: glm/test/core/core_type_length.cpp

[Groovounet]

This issue is fixed in master branch and GLM 0.9.8.2 release.

[elect86]

I'd need also a parameter to return, for matrices, the number of elements

For a mat4 this means 16, where length actually return 4 (because of 4 columns)

It's not urgent at all, it's just sugar code, but it'd nice to have