Merge pull request #2382 from jayschwa/master

Manual: Replace deprecated `strcat` with `string`
doc: replace more deprecated "strcat" uses with "string" in examples.
always expand @mstr before other macros

This is probably the wrong way to do this, but it fixes the following
problem/inconsistency:

    julia> E"""$n"""
    ERROR: no method unescape_string(Expr,)

    julia> @printf "%s" E"""$n"""
    $n