Extend macros to be able to copy a value from another macro

[delphidabbler]

For example there may be cases, particularly if issue #68 is implemented (conditional compilation), where it is useful to set one macro value from another.

For example, say we have the macro

Include:data=info.dat

with contents

one=1
two=two
some-long-name=42

then we have to refer to some-long-name as data.some-long-name within the .vi file.

It may be clearer to use another variable for that name, such as answer. So having the set macro type would be useful:

set:answer=data.some-long-name

so that answer would now be set to 42, the value of data.some-long-name.

Whether the data should be copied or whether the new name is a simple alias needs some thought.

[delphidabbler]

If the new name is a simple alias then a better name for the macro type would be Alias:

alias:answer=data.some-long-name

Conversely, if the intention is to copy one value to another then Let may be a better name than Set, which could be confused with a mathematical set:

let:answer=data.some-long-name