More specialized routines for Triangular matrices

This branch provides additional specialized routines for Triangular matrices

- Generic linear solver using naive backward/forward substitution (implemented in `naivesub!`)
- Generic matrix inversion using linear solver on identity matrix
- `cond`, condition number estimate, wrapping
- `LAPACK.trcon!`, reciprocal of condition number estimate for triangular matrices
- tests for linear solver and cond()
- specialized `transpose`, `ctranspose`, `istril`, `istriu`

Additionally:
- Additional `zeros`, `ones`, `infs` and `nans` methods taking an AbstractMatrix argument
- Type-stability bugfixes for generic `cond` method on dense matrices