title: convert character linestring to geometry in sf
body: <p>Basic question, how do I convert type <code>character</code> <code>LINESTRING</code> variable to a <code>geometry</code> type in <code>sf</code> and then separately, separate the two coordinates into different variables. E.g.</p>
  
  <pre><code>library(tidyverse)
  library(sf)
  df &lt;- structure(list(geometry = c("LINESTRING (-85.76 38.34, -85.72 38.38)", 
                              "LINESTRING (-85.46 38.76, -85.42 38.76)", 
                              "LINESTRING (-85.89 38.31, -85.85 38.32)"
  ), var1 = c(4, 5, 6
  ), var2 = c(1, 2, 3
  )), row.names = c(NA, -3L), class = c("tbl_df", "tbl", "data.frame"
  ))
  df
  # # A tibble: 3 x 3
  #   geometry                                 var1  var2
  #   &lt;chr&gt;                                   &lt;dbl&gt; &lt;dbl&gt;
  # 1 LINESTRING (-85.76 38.34, -85.72 38.38)     4     1
  # 2 LINESTRING (-85.46 38.76, -85.42 38.76)     5     2
  # 3 LINESTRING (-85.89 38.31, -85.85 38.32)     6     3
  </code></pre>
  
  <p><strong>Desired output:</strong></p>
  
  <pre><code>final
  #                     geometry   start_lon start_lat end_lon end_lat var1 var2
  # -85.76, -85.72, 38.34, 38.38      -85.76     38.34  -85.72   38.38    4    1
  # -85.46, -85.42, 38.76, 38.76      -85.46     38.76  -85.42   38.76    5    2
  # -85.89, -85.85, 38.31, 38.32      -85.89     38.31  -85.85   38.32    6    3
  
  
  #so I can quickly plot lines between points
  plot(final[, "var1"])
  </code></pre>
  
  <p>I thought some combination like  <code>final &lt;- st_as_sf(df) %&gt;% st_geometry(geometry)</code> would convert it but I cant figure it out. Perhaps <code>sfheaders</code> is useful here?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>I want an <code>sf</code> type solution and not to manually extract coordinates.</p>
  
  <p>Any suggestions?
  thanks</p>