PHP7 compatibility

[Hywan]

Hey :-),

PHP7 introduces some BC breaks and we have to track and fix them before end of June.
Here is the official list that changed: https://github.com/php/php-src/blob/master/UPGRADING.

How to track them?

1. run hoa test:run --all often,
2. run

find . -name "Bool\.php" -o \
    -name "Int\.php" -o \
    -name "Float\.php" -o \
    -name "String\.php" -o \
    -name "Null\.php" -o \
    -name "False\.php" -o \
    -name "True\.php" -o \
    -name "Resource\.php" -o \
    -name "Object\.php" -o \
    -name "Mixed\.php" -o \
    -name "Numeric\.php"

until it results nothing,

1. track some place where BaseException must be used instead of Exception,
2. add retro-compatibility with prior versions of PHP7.