Precompile instructions for basic algebra operations

[jishnub]

A simpler alternative to #283, without the need for extra packages, and one that seems to work almost as well for many cases.

After this, the time to first execution practically disappears for addition and subtraction for the common use case where the parent is a Matrix{Float64}

julia> using BandedMatrices

julia> B = BandedMatrix(0=>ones(1000));

julia> @time B + B;
  0.000110 seconds (63 allocations: 11.375 KiB)

Some others aren't completely eliminated, but considerably improved:

julia> v = ones(size(B,2));

julia> @time @eval B * v;
  0.009193 seconds (8.87 k allocations: 556.567 KiB)

[codecov]

Codecov Report

Base: 80.55% // Head: 80.55% // No change to project coverage 👍

Coverage data is based on head (033dd22) compared to base (dfbf44f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #292   +/-   ##
=======================================
  Coverage   80.55%   80.55%           
=======================================
  Files          23       23           
  Lines        3270     3270           
=======================================
  Hits         2634     2634           
  Misses        636      636           

Impacted Files	Coverage Δ
src/BandedMatrices.jl	100.00% <ø> (ø)

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