How to get a parseable unit string

I have the following problem:

```
using Unitful
# Variable with unit defined
v = 2.0u"m/s"

# Extract unit as string
v_unit = string(unit(v)) # = "m s^-1"

# This string cannot be parsed
uparse(v_unit) # gives an error

# Therefore code generation with the v_unit string does not work
code = :( @u_str($v_unit) ) # = :( u"m s^-1" )
eval(code) # Gives an error
```

The reason is that string(unit(v)) returns a string that cannot be parsed. In Julia this seems to be an unusual behavior because string(*something*) typically returns a string representation of *something* that can be again parsed by Julia.

I searched extensively in Unitful documentation and the Issues but did not find a solution.

I currently use a bad hack (that most likely does not work in all situations), by replacing " " by "*":

```
v_unit = replace( string(unit(v)), " " => "*") # = "m*s^-1"
uparse(v_unit) # fine
code = :( @u_str($v_unit) ) # = :( u"m*s^-1" )
eval(code) # fine
```

Help is appreciated

Start by reproducing the issue with string(unit(v)), uparse, and the @u_str code-generation example shown in the report. Read the implementations or tests for unit stringification and parsing; done means a unit string produced from a unit value can be parsed and used in the demonstrated generated-code path without the replacement hack.

Written by the indexing model from the issue text.