Add a !global construct

[suve]

Currently functions have unfettered access to variables in higher scopes - be it the program's globals, or the caller's variables. This is a really stupid design decision that should be changed.

Proposed solution:

• Limit functions to accessing only their local variables (and args/statics)
• To use global variables, one must explicitly declare their use with !global $varname
• The construct should probably be limited to the function's top-level scope (can't be put inside !if or loops)

Things to consider:

• Should !global $var and !global &var be equivalent, or should the first one create a copy of the global variable in local scope (instead of adding a reference)?
• What should happen when a function calls !global &var and &var hasn't been yet set?

[suve]

Partially done in ad7d725. Currently there is no difference between !global $var and !global &var. The construct can be used inside !if and loops to make it consistent with !static.