FEAT: added paren! support in paths for compiler.
FEAT: added REDUCE native in compiler. /into option implemented.

/into will append the reduced values to the argument block instead of inserting them. The reason is that INSERT action has not yet been implemented.
FEAT: added REDUCE native support to interpreter.
FEAT: more general-purpose `eval-next` function.
FEAT: now REDUCE supports non-literal block argument (both in compiler and interpreter).
TESTS: initiated the interpreter tests.
FIX: wrong code generated by compiler for empty block! series passed to REDUCE.
TESTS: added REDUCE tests.
TESTS: additional tests for nested REDUCE calls.
FIX: removed unused argument in EVAL calls.
FIX: block/rs-head not taking block offset into account.
FEAT: added various 'append functions to block! internal API.
FEAT: added COMPOSE native to interpreter. Supports /deep, /only and /into options.

Reminder: refinements are not yet supported by interpreter!
FIX: wrong types for options passed to COMPOSE in interpreter.
FIX: extra whitespace in negative number (thanks to HostileFork for reporting it).